Democrats and Republicans are locked in a standoff over the terms of U.S. President Donald Trump's impeachment trial in the Senate, with House Speaker Nancy Pelosi refusing to formally hand over the articles of impeachment until Republican plans become clear.

What do the sides want?

Democrats pressed the Republican-controlled Senate on Thursday to call Trump's top lieutenants to testify, as they sought to focus attention on the trial ahead of the 2020 presidential election.

Senate Republican Majority Leader Mitch McConnell said he met with Senate Democratic leader Chuck Schumer but they failed to reach an agreement.

Democrats want McConnell to allow top Trump aides like Mick Mulvaney, the White House acting chief of staff, and John Bolton, Trump's former national security adviser, to testify, according to a senior Democratic aide.

"Is the president's case so weak that none of the president's men can defend him under oath?" asked Senate Democratic Leader Chuck Schumer, who later urged McConnell in a meeting to use the two-week recess to consider allowing witnesses.

What do Americans want?

Fewer than half of Americans want Trump removed from office, according to a Reuters/Ipsos poll released on Thursday.

When asked about the specific impeachment charges, 53 percent agreed that Trump abused his office and 51 percent agreed that he obstructed Congress. 

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Some 42 percent of respondents – most of them Democrats – said Congress should remove the president from office. 

Another 17 percent said Trump should be formally reprimanded with a congressional censure, 29 percent said they want the impeachment charges dismissed, and the rest said they did not have an opinion.

What happens next?

The Senate trial is expected in early January. Trump himself has expressed an interest in a long trial with witnesses, but senior Republican senators want to put the affair behind them.

McConnell and Schumer met on Thursday afternoon. Asked how he felt about Pelosi potentially withholding the articles of impeachment, the Republican said: "If the speaker wants to hold on to them it's fine with us."

Earlier, McConnell accused Democrats of succumbing to "transient passions and factionalism" and made it clear that he did not think the Senate should find Trump guilty.

"The vote did not reflect what had been proven. It only reflects how they feel about the president. The Senate must put this right," McConnell said on the Senate floor.

The Senate is highly unlikely to remove Trump from office. At least 20 Republican senators would have to vote to convict Trump and so far none have indicated they are open to doing so.

What's the background?

In a historic vote on Wednesday, House Democrats impeached Trump for alleged abuse of power and obstruction of Congress related to his attempts to pressure Ukraine to investigate Democratic political foe Joe Biden.

He is only the third U.S. president to be impeached.

Trump is accused of abusing his power by pressuring Ukraine to investigate Biden, as well as a discredited theory that Democrats conspired with Ukraine to meddle in the 2016 election.

Then 73-year-old is also accused of obstruction of Congress for directing administration officials and agencies not to cooperate with the impeachment inquiry.

A Senate trial would kick off a politically charged year heading into the presidential election, which will pit Trump against one of a field of Democratic contenders, including Biden, who have repeatedly criticized Trump's conduct in office.

(With input from agencies)