WASHINGTON (REUTERS) - Top congressional Democrats left the door open on Sunday (April 21) to the impeachment of United States President Donald Trump, but said they would first need to complete their own investigations into whether he obstructed justice in Special Counsel Robert Mueller's Russia probe.

Some party leaders have cautioned against impeachment just 18 months before the 2020 presidential election, although prominent liberals have demanded the start of proceedings to remove Mr Trump from office since the release of a redacted version of Mr Mueller's report last Thursday.

US House Judiciary Committee chairman Jerrold Nadler, whose panel would spearhead any impeachment proceedings, said Democrats would press ahead with investigations of Mr Trump in Congress and "see where the facts lead us".

"Obstruction of justice, if proven, would be impeachable," Mr Nadler said on NBC's Meet the Press.

The redacted version of Mr Mueller's long-awaited report on Russian interference in the 2016 election, the product of a 22-month-long investigation, outlined multiple instances where Mr Trump tried to thwart the probe. While it stopped short of concluding that Mr Trump had committed a crime, it did not exonerate him.

Mr Mueller also noted that Congress has the power to address whether Mr Trump violated the law, and Democrats said it would be a matter of discussion in the coming weeks.

"That's going to be a very consequential decision and one I'm going to reserve judgment on until we have a chance to fully deliberate on it," House Intelligence Committee chairman Adam Schiff said on Fox News Sunday.

Democrats in the House of Representatives planned a conference call for Monday afternoon to discuss their next steps in response to the Mueller report. Democratic House Speaker Nancy Pelosi wrote to lawmakers last week to notify them of the call "to discuss this grave matter".

Mr Nadler has issued a subpoena to the Justice Department to hand over the full, unredacted Mueller report and other relevant evidence by May 1, although the Justice Department called the request "premature and unnecessary".

Before drawing any conclusions, Mr Nadler said Democrats would want to see those materials as well as hear from Mr Mueller and Attorney-General William Barr, who is scheduled to testify in early May. Mr Nadler also said he would call former White House counsel Donald McGahn to testify.

Republicans have stood by Mr Trump, and Speaker Pelosi has cautioned against an impeachment effort that would have no chance of success in the Republican-led Senate.

US Senator Elizabeth Warren became the first major contender for the Democratic 2020 presidential nomination to call for the start of impeachment, saying last Friday that "the severity of this misconduct" demanded it.

Mr Julian Castro, former housing secretary under president Barack Obama and another 2020 contender, joined Ms Warren in backing the launch of impeachment proceedings.

Democratic House Oversight Committee chairman Elijah Cummings said on CBS television's Face The Nation that he could foresee possible impeachment proceedings against Mr Trump, "but I'm not there yet".

He also said Congress needed to look at Mr Trump's finances and gauge Mr Mueller's intentions with his report. But even if Senate Republicans blocked a Democratic impeachment effort, Mr Cummings said: "I think history would smile upon us for standing up for the Constitution."

Representative Tim Ryan, another Democratic presidential contender, said the party should wait until multiple ongoing investigations of Mr Trump in Congress have had a chance to uncover more evidence.

"Let the process play itself out," he said on CNN's State Of The Union programme. "I would just rather us take this next step: educate the American people, really get these details out, let the Judiciary Committee do its work."

Mr Trump, who has repeatedly called the investigation a "witch hunt", has claimed vindication from Mr Mueller's report. Mr Rudy Giuliani, one of Mr Trump's lawyers, sought to undermine the credibility of Mr Mueller's investigators on NBC's Meet The Press.

"I don't think his people are fair," Mr Giuliani said of Mr Mueller's team. "I don't think that report is fair."

While Mr Trump's team had indicated it would release a rebuttal to Mr Mueller's report, Mr Giuliani said that it was not imminent although it would probably be released at some point.

"We planned to do it if we needed to. So far, we don't think we need to," he said on Fox News Sunday.