WASHINGTON - President Donald Trump on Tuesday accused Democrats of seeking to avoid an impeachment trial in the Senate to protect Joe Biden and his son Hunter, whose service on the board of a Ukrainian energy company while his father was vice president has come under scrutiny.

Trump weighed in on Twitter as an impasse continued over the timing and scope of an expected Senate trial. House Speaker Nancy Pelosi, D-Calif., has held back the two articles of impeachment - abuse of power and obstruction of Congress - passed this month by the House.

She and Senate Democrats are seeking guarantees that a Senate trial will include several witnesses, notably acting White House chief of staff Mick Mulvaney and former national security adviser John Bolton, who declined to participate in the House impeachment proceedings. Democrats also are pushing for the release of documents relevant to Trump's conduct toward Ukraine that the White House has refused to provide.

"The Democrats will do anything to avoid a trial in the Senate in order to protect Sleepy Joe Biden, and expose the millions and millions of dollars that 'Where's' Hunter, & possibly Joe, were paid by companies and countries for doing NOTHING," Trump said in his tweet. "Joe wants no part of this mess!"

Trump has previously insisted that a Senate trial include the Bidens as witnesses, along with the anonymous whistleblower whose complaint sparked the impeachment inquiry.

Later Tuesday, Trump took direct aim at Pelosi.

"Remember when Pelosi was screaming that President Trump is a danger to our nation and we must move quickly," he tweeted. "They didn't get one Republican House vote, and lost 3 Dems. They produced no case so now she doesn't want to go to the Senate. She's all lies. Most overrated person I know!"

While records suggest that Hunter Biden was paid lucratively for his service on the board of the Burisma energy company, no evidence has emerged of any wrongdoing. There has also been no evidence that Joe Biden, now a Democratic presidential candidate, was paid in connection with his son's service on the board. As part of his duties as vice president, Joe Biden was heavily involved in U.S. policy toward Ukraine.

As Trump tweeted from his Mar-a-Lago estate in Palm Beach, Fla., on New Year's Eve, it remained unclear when a trial might start in the Republican-led Senate.

Senate Majority Leader Mitch McConnell, R-Ky., is insisting that senators hear opening statements by House impeachment managers and lawyers for Trump before deciding whether to allow either side to call witnesses. McConnell has also indicated that it would be fine with him if Pelosi never transmits the articles of impeachment and a trial is not held.

At the heart of the Democrats' case is the allegation that Trump tried to leverage a White House meeting and military aid, sought by Ukraine to combat Russian military aggression, to pressure Ukrainian President Volodymyr Zelensky to launch an investigation of Joe Biden and Hunter Biden, as well as a probe of an unfounded theory that Kyiv conspired with Democrats to interfere in the 2016 presidential election.

Senate Minority Leader Chuck Schumer, D-N.Y., continued late Monday night to push his party's case for calling witnesses.

"So far, neither Senator McConnell nor any Republican Senator has articulated a single good reason why the trial shouldn't have the witnesses or the documents we requested," he said in a tweet. "The American people deserve to know the truth."

Meanwhile, some House Democrats pressed the argument Tuesday that Pelosi should hold onto the articles of impeachment as long as necessary.

"We shouldn't send them until we have assurances it's going to be a fair and honest trial," Rep. Dan Kildee, D-Mich., said during an appearance on CNN, adding that he wants "assurances that the trial isn't going to be a sham."

To secure the witnesses they seek without a deal with McConnell, Senate Democrats would need to persuade at least four Republican senators to vote with them to call them.

On Monday, one of those whom Democrats would likely court, Sen. Susan Collins, R-Maine, indicated in a radio interview that she favors the process McConnell has outlined.

Collins told Maine Public that she is "open to witnesses" but thinks that decision should be made after opening arguments and an opportunity for senators to present written questions to both sides.

"I think it's premature to decide who should be called until we see the evidence that is presented and get the answers to the questions that we senators can submit through the Chief Justice to both sides," Collins said, referring to Supreme Court Chief Justice John Roberts, who would preside over a Senate trial.

Collins was also critical of McConnell for saying he is closely coordinating trial preparations with the White House, echoing earlier criticism from another GOP moderate, Sen. Lisa Murkowski of Alaska.

"It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us, because the each of us will take an oath, an oath that I take very seriously to render impartial justice," Collins said.

"That's what it says, impartial justice. And I have heard Democrats like Elizabeth Warren, saying that the president should be impeached, found guilty, and removed from office. I've heard the Senate majority leader saying that he's taking his cues from the White House. There are senators on both sides of the aisle, who, to me, are not giving the appearance of and the reality of judging that's in an impartial way."

Lawmakers are scheduled to return to Washington next week.

Biden struggled over the weekend to explain his position on what he would do in response to a subpoena to testify in a Senate trial. For much of Saturday, he suggested he would defy such a subpoena, before changing course and saying he would abide by "any subpoena that was sent to me."

That latter comment came even as Biden continued to insist there is no "legal basis" for him to be called as a witness in the impeachment proceedings.

Democrats have argued that the issue of the Bidens' involvement in Ukraine is irrelevant to the question of whether Trump abused his power when he asked Zelensky to launch an investigation into the Bidens.

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The Washington Post's Felicia Sonmez and Colby Itkowitz contributed to this report.