WASHINGTON (BLOOMBERG) - Former President Donald Trump's lawyers in his unprecedented second impeachment trial closed their defence presentation arguing that the charge brought by the Democratic-controlled House is an attempt to stifle speech and has no constitutional basis.

"Let's be clear, this trial is about far more than President Trump. It is about silencing and banning speech the majority does not agree with," attorney Bruce Castor said, echoing the "cancel culture" theme often voiced by Trump and his supporters.

"It is about cancelling 75 million Trump voters and criminalising political viewpoints."

Trump's team spent less than three of the 16 hours they were alloted in presenting his defence, primarily arguing that his incendiary rhetoric during a Jan 6 speech in Washington before his supporters stormed the Capitol was ordinary political speech and fell far short of the legal standard for incitement.

The trial next proceeds to a question and answer period of up to four hours in which senators can pose written queries to each side.

Democrats have portrayed Trump's call for his supporters to "fight like hell" before they broke into the Capitol to stop the counting of Electoral College votes as the then-president's last-gasp bid to stay in power. The House impeachment mangers spent two days portraying Trump as lawless and unrepentant, arguing that he should not hold public office again.

But lawyer Michael van der Veen said Trump never encouraged "unlawful activity of any kind" and the protest rally he addressed was hijacked by "a small group" determined to wreak violence.

"The article of impeachment now before the Senate is an unjust and blatantly unconstitutional act of political vengeance," van der Veen said.

"This appalling abuse of the Constitution only further divides our nation when we should be trying to come together around shared priorities."

Trump's defence team used a lengthy video of numerous Democratic politicians, including many on the prosecution team, using the words "fight" or "fighting" in political speeches.

They also showed Democrats challenging electoral votes after several recent presidential elections. They featured lawmakers, including lead impeachment manager Jamie Raskin, objecting to the counting of votes in January 2017 after Trump's election, and President Joe Biden, then in his role as vice-president, repeatedly gaveling them down as out of order.

Trump lawyer David Schoen decried "the hatred, the vitriol, the political opportunism that has brought us here today."

He blamed the impeachment on "hatred, animosity, division, political gain - and let's face it, for House Democrats, President Trump is the best enemy to attack."

Senator John Thune, the No. 2 GOP leader, said he questions whether the strategy of showing videos of Democrats using inflammatory language would be effective.

The South Dakota lawmaker told reporters, "I'm not sure it bears a lot on this case" and the defence team should address Trump's actions on Jan 6 and during the Capitol attack.

The trial is all but certain to end with Trump's acquittal. A conviction would require 17 Senate Republicans to join with Democrats and independents in finding Trump guilty to reach the two-thirds majority necessary. Even the Senate's No. 2 Democrat said he saw little indication that many Republicans would back conviction.

"Many of them are loyal to Donald Trump even to this day, despite what he may have said about them or their families in the past," Senator Dick Durbin of Illinois said in an interview on Thursday with Bloomberg Television. "And more of them are afraid of Donald Trump's political power."

Biden told reporters he was "just anxious to see what my Republican friends do, if they stand up," adding that he had no plans to call GOP senators to discuss their votes.

Schoen met for about an hour Thursday night with several Republican senators, who are also jurors in the trial.

Trump adviser Jason Miller said during interview with Newsmax on Friday that Ted Cruz of Texas, Lindsey Graham of South Carolina and Mike Lee of Utah in a meeting with Schoen had "some very good ideas" pertaining to Trump's defence and that "it was a real honour to have those senators come in and give us some additional ideas, so we're appreciative for that."

House managers meticulously highlighted over two days Trump's own tweets, speeches and comments to argue that his months-long campaign to stoke anger about the Nov 3 election he lost to Biden - including attempts to overturn the results with "the big lie" that the vote was stolen - inevitably culminated in the riot he did nothing to prevent or stop.

"Senators, America, we need to exercise our common sense about what happened," Raskin said in the Democrats' final arguments. "Let's not get caught up in a lot of outlandish lawyers' theories here. Exercise your common sense about what just took place in our country."

While Schoen called the process partisan, Democrats have cited condemnations of Trump's actions by Republicans, including the 10 House members, led by Representative Liz Cheney, one of the chamber's GOP leaders, who voted to impeach him.

The defence will likely also have to contend with some of the unanswered questions posed by Raskin, including why Trump did not act to end the insurrection until hours after it started.

Senator Bill Cassidy, the Louisiana Republican who criticised Trump's lawyers' arguments against the constitutionality of the trial earlier in the week, told reporters he wants to hear the defence explain Trump's lack of action once the mob breached the Capitol.

Besides arguing the trial is unconstitutional and that Trump did not incite the violence, his lawyers said the impeachment violates his First Amendment protection and that the quick impeachment by the House disregards his due process rights.

Democrats said on Thursday in advance of those arguments that they are without merit and an attempt to distract from the facts.

In his closing argument on Thursday, Raskin implored Trump's attorneys not to dwell on the argument that the trial is unconstitutional because Trump is no longer in office.

The Senate voted 56-44 on Tuesday - with six Republicans joining all Democrats and two independents - that the Senate can legally try the case after both sides presented their arguments.