The Democratic-led United States House of Representatives will vote on Wednesday to send formal impeachment charges against President Donald Trump to the Senate, lawmakers said on Tuesday, bringing the start of Trump's trial one step closer.

House Speaker Nancy Pelosi told a party meeting that she would also name the House "managers" who will lead the prosecution of Trump at the trial, Democratic Congressman Henry Cuellar said.

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The House impeached Trump last month on charges of abusing power by pressuring Ukraine to announce an investigation into his Democratic rival Joe Biden and charges of obstructing Congress.

But Pelosi has delayed sending the charges to the Senate in an effort to get that chamber's h to agree to include new witness testimony that could be damaging to the Republican president. McConnell says he has the votes to start the impeachment trial without an agreement on witnesses.

US Speaker of the House Nancy Pelosi answers questions during a news conference at the US Capitol [Win McNamee/Getty Images/AFP]

A Wednesday vote would allow the Senate to begin the trial as soon as this week. However, opening arguments will likely not be heard until next week at the earliest, as the Senate will likely take several days to get through formalities such as swearing in members.

Trump has denied any wrongdoing and has dismissed his impeachment as a partisan bid to undo his 2016 election win as he faces a re-election contest in November.

He is only the third US president to be impeached. No president has been removed from office as a result of an impeachment trial.

A motion to dismiss?

The Senate is expected to acquit Trump, as no Republicans have voiced support for removing him, a step that would require a two-thirds majority. But some Republican senators are conferring privately about whether to allow a motion to dismiss the charges against the president or to call additional witnesses for testimony.

Trump suggested over the weekend that he might prefer simply dismissing the charges rather than giving legitimacy to charges from the House, which he considers a "hoax".

It was an extraordinary suggestion, but one being proposed by Trump allies with support from some Republican senators, including McConnell.

But it is clear McConnell does not have the 51 votes needed from his Republican majority to do that.

Key Republicans, Senator Susan Collins and Senator Mitt Romney, said would oppose a motion to dismiss the charges.

Witnesses?

Collins is leading an effort among some Republicans, including Romney and Senator Lisa Murkowski of Alaska, to ensure the ground rules include plans to eventually consider voting to call witnesses.

"My position is that there should be a vote on whether or not witnesses should be called," Collins said.

Romney said he wants to hear from John Bolton, the former national security adviser at the White House, who others have said raised alarms about the alternative foreign policy towards Ukraine being led by Trump's personal lawyer Rudy Giuliani.

"I've said I'd like to hear from John Bolton," Romney told reporters Monday. "I expect that barring some kind of surprise, I'll be voting in favour of hearing from witnesses after those opening arguments."

Democrats have been pushing Republicans, who have the majority in the Senate, to consider new testimony, arguing that fresh information has emerged during Pelosi's delay in transmitting the charges.

McConnell is drafting an organising resolution that will outline the steps ahead. Approving it will be among the first votes that senators take after they are sworn as jurors by Supreme Court Chief Justice John Roberts for the court of impeachment.

McConnell is hesitant to call new witnesses who would prolong the trial. He prefers to model Trump's trial partly on the process used for then-President Bill Clinton's trial in 1999.

Senate Majority Leader Mitch McConnell speaks with reporters after walking off the Senate floor [File: Patrick Semansky/AP Photo]

Senators say if witnesses are allowed, some Republicans may also try to subpoena Biden and his son, Hunter Biden, who served on the board of a gas company in Ukraine while his father was vice president.

There has been no evidence of wrongdoing by the Bidens.

It takes just 51 votes during the impeachment trial to approve rules or call witnesses. Just four Republican senators could form a majority with Democrats to insist on new testimony.

Most Republicans appear willing to go along with McConnell's plan to start the trial first then consider witnesses later, rather than upfront, as Democrats want.

Collins is pushing to have at least the promise of witness votes included in the organising resolution. She and the others appear to be gathering support.

"I've been working to make sure that we will have a process that we can take a vote on whether or not we need additional information, and yes, that would include witnesses," Murkowski told reporters.