WASHINGTON (NYTIMES) - US President Donald Trump's legal team called on the Senate on Monday (Jan 20) to "swiftly reject" the impeachment charges and acquit him, arguing that Democrats would "permanently weaken the presidency" if they succeeded in removing him from office over what the team characterised as policy and political differences.

In a 110-page brief submitted to the Senate the day before Mr Trump's trial begins in earnest, the president's lawyers advanced their first sustained legal argument since the House opened its inquiry in the fall, contending that the two charges approved largely along party lines were constitutionally flawed and set a dangerous precedent.

Mr Trump's lawyers dismissed the validity of both articles of impeachment lodged against him - abuse of power and obstruction of Congress - because they do not state any specific violation of the law, advancing a constrained and widely rejected interpretation of the power to impeach a president.

While the lawyers did not contest the basic facts of the case, they maintained that Democrats' accusations in effect seek to punish the president for foreign policy decisions and efforts to preserve executive prerogatives.

"They do not remotely approach the constitutional threshold for removing a president from office," the brief said.

"The diluted standard asserted here would permanently weaken the presidency and forever alter the balance among the branches of government in a manner that offends the constitutional design established by the founders."

In their own detailed legal brief submitted Saturday, the House impeachment managers outlined their case that Mr Trump corruptly solicited foreign interference in the 2020 election for his own benefit by pressuring Ukraine to announce investigations into his political rivals while withholding nearly US$400 million (S$538.68 million) in security aid the country desperately needed as well as a coveted White House meeting for its president.

"President Trump did not engage in this corrupt conduct to uphold the presidency or protect the right to vote," the seven House Democratic impeachment managers said Monday in a second filing that rebutted many of the president's assertions.

"He did it to cheat in the next election and bury the evidence when he got caught."

"Mr Trump's answer to the charges offers an unconvincing and implausible defence against the factual allegations in Article I," the managers wrote. "The 'simple facts' that it recites confirm President Trump's guilt, not his innocence."

The legal brief filed by his lawyers did not deny that Mr Trump asked Ukraine to announce the investigations into Democrats, including former vice-president Joe Biden, nor that he withheld military aid that Congress had approved for Kyiv.

But his lawyers said that the president never tied the investigations to a White House meeting or the security assistance.

And in any case, they argued that the president has the right to conduct relations with other countries as he sees fit and that he had valid reasons to raise those issues with Ukraine because he wanted to root out corruption there and get other countries to share the burden of providing military aid.

The lawyers dismissed the notion that doing so was an abuse of power, as outlined in the first article of impeachment, calling that a "novel theory" and a "newly invented" offence that would allow Congress to second-guess presidents for legitimate policy choices.

"House Democrats' concocted theory that the president can be impeached for taking permissible actions if he does them for what they believe to be the wrong reasons would also expand the impeachment power beyond constitutional bounds," the brief said.

"It is the president who defines foreign policy," it added, and said that Mr Trump had "legitimate concerns" in raising the issues involving Democrats with the Ukrainians.

The lawyers argued that the second article, accusing Mr Trump of obstructing Congress by blocking testimony and refusing to turn over documents during the House impeachment inquiry, was "frivolous and dangerous" because it would invalidate a president's right to confidential deliberations in violation of the separation of powers.

In making their case, the White House lawyers themselves embraced novel interpretations of the history of impeachment. Far from newly invented, the concept of abuse of power was envisioned by the framers from the start. Mr Alexander Hamilton specifically described impeachment as a remedy for the "abuse or violation of some public trust."

The House Judiciary Committee adopted articles of impeachment accusing both Presidents Richard Nixon and Bill Clinton of abuse of power for failing to properly respond to demands for information by Congress.

In neither case was the allegation eventually approved by the full House because Nixon resigned first and the House voted down that article against Mr Clinton. Neither of them dealt with the conduct of foreign policy, so that element is different.

Many constitutional scholars have long said that impeachable offences do not have to be specific violations of a criminal code, but could be broader violations of a president's oath of office or offenses against the republic.

In the case of President Andrew Johnson, one of the articles against him alleged no violation of law but impeached him anyway for speeches bringing Congress into "disgrace, ridicule, hatred, contempt and reproach."

While the White House brief argued that the articles against Mr Trump did not allege an actual crime, a report released last week by the Government Accountability Office, an independent, non-partisan government agency, found that the Trump administration violated the law by withholding the security aid allocated by Congress.

The decision to freeze the aid was directed by the president himself, and during the House inquiry, administration officials testified that they had raised legal and policy concerns that were ignored.

The president's legal team took issue with the Government Accountability Office's conclusion and said that, in any case, it was irrelevant because it was not included in the articles of impeachment themselves.

The White House brief stressed that Mr Trump ultimately met with President Volodymyr Zelenskiy and released the aid even though the Ukrainians never announced the investigations the president had sought.

But the money was delivered and the meeting set only after a whistleblower had filed a complaint alleging impropriety by the president and lawmakers had opened their own investigation into why the money had been blocked.

The dueling filings rolled in as both sides braced for a contentious trial on the Senate floor over whether to remove Mr Trump, only the third such impeachment proceeding in the country's history.

Mr Trump is set to leave Monday for an economic conference in Davos, Switzerland, so he will be out of the country meeting with other world leaders as the Senate begins to weigh his fate.

In the Capitol, the House managers and the president's defence team took turns Monday privately touring the Senate chamber and surrounding offices, transformed over the weekend into a court of impeachment that will open Tuesday with a debate on the rules for the trial.

Oral arguments by the House managers are expected to begin as early as Wednesday, followed by a presentation by Mr Trump's team.

The president's lawyers used their brief to revive complaints about the House impeachment process, calling it "rigged."

They made a point of quoting numerous Democrats who had denounced partisan impeachments in the past, either during the Clinton era or more recently. Among others, they cited Mr Biden and House Speaker Nancy Pelosi, who just last year said impeachment should be pursued only if it were "clearly bipartisan."

On Monday, House Democrats sought to dismantle the president's case. In arguing that abuse of power is not an impeachable offense, they said, Mr Trump's lawyers were ignoring the intentions of the founders and in effect asserting that "the American people are powerless to remove a president for corruptly using his office to cheat in the next election."

The managers also said the president's attempt to justify his obstruction failed to account for the House's broad prerogative to conduct their inquiry. The House investigation was "properly authorised," they insisted, and they pointed out that Mr Trump never actually legally invoked executive privilege, but merely raised the threat of doing so to discourage officials from testifying.

The nine-page filing was technically a response to a shorter pleading submitted by Mr Trump's team Saturday, but it previewed how the House intends to deconstruct the president's defense.

Behind closed doors, the House impeachment managers met in Ms Pelosi's Capitol office suite, preparing the presentations they will deliver this week to try to persuade senators to call witnesses in the trial and, ultimately, to convict Mr Trump.

The president's lawyers, including Mr Pat Cipollone and Mr Jay Sekulow, travelled from the White House to inspect the Senate chamber.

"We anticipate being able to make a vigorous case as quickly as possible," Mr Eric Ueland, the White House legislative affairs director, told reporters.

The president weighed in himself, complaining that he had not been treated fairly and dismissing demands by Senator Chuck Schumer of New York, the Democratic leader, for a trial that would include witnesses and testimony that the president has so far blocked.

"Cryin' Chuck Schumer is now asking for 'fairness', when he and the Democrat House members worked together to make sure I got ZERO fairness in the House," Mr Trump wrote on Twitter. "So, what else is new?"

He specifically mocked suggestions that the Senate call Mr John Bolton, the former national security adviser, as a witness in the trial.

"They didn't want John Bolton and others in the House," Mr Trump tweeted. "They were in too much of a rush. Now they want them all in the Senate. Not supposed to be that way!"

In fact, House Democrats did want Mr Bolton to testify in their proceedings, but he refused, citing Mr Trump's insistence that his current and former advisers not participate.

House Democrats did not issue a subpoena or otherwise pursue the matter, concluding that a lengthy, drawn-out court fight to compel Mr Bolton's testimony would drag out the matter for months. Mr Bolton has since said he would testify in a Senate trial if subpoenaed.

The trial was set to begin on an acrimonious note Tuesday, when Republicans are expected to muscle through their rules along party lines. Senator Mitch McConnell, the Senate majority leader, had yet to release the measure by Monday afternoon, leaving the House managers and Senate Democrats grumbling and mostly in the dark about what was to come.

Modeled on the procedures for Mr Clinton's 1999 trial, a draft of the resolution affords the managers and the president's lawyers 24 hours each to present their case.

Unlike that earlier trial, however, this time Republicans are contemplating insisting that the prosecution and defence use their time over just two marathon days - allowing the Senate to get through opening arguments this week.

If it did, senators could then pose questions to the two sides next week before debating whether to call witnesses or seek documents. Democrats plan to oppose the procedures on Tuesday because they want a guarantee of witnesses at the outset, arguing that anything less constitutes an attempt to rush an acquittal.

A draft of the resolution also proposes giving Mr Trump's lawyers the right to make a motion early in the trial, before opening arguments, to dismiss the case altogether, according to Republicans who have reviewed it.

That presents Mr Trump's legal team with a dilemma. The president has called for an outright dismissal. But with moderate Republicans insistent on hearing the case out, any motion to dismiss would almost certainly fail in a vote on the Senate floor, dealing Mr Trump a setback at the outset of the trial.