U.S. President Donald Trump listens during a meeting about the Governors Initiative on Regulatory Innovation in the Cabinet Room of the White House on 16 December 2019 in Washington, DC. (AFP)

US Democrats signaled Monday they were girding for battle over witnesses and fair process in the Senate trial of Donald Trump, days ahead of a historic House vote on impeaching the president for abuse of office and obstruction of Congress.

Even as Trump faces disgrace given the likelihood of his impeachment, a new poll shows his approval rating has inched upwards to a high point, although the figure remains underwater.

Top Senate Democrat Chuck Schumer said he was aiming for proceedings to start January 6 that would mete out “swift but fair justice” to Trump, though Republican loyalists acknowledged they were less interested in being impartial jurors than protecting the president.

Lawmakers were beginning a consequential week. House Judiciary Committee chairman Jerry Nadler released his panel’s final report Monday outlining the case for impeaching Trump and detailing his alleged wrongdoing, including pressuring Ukraine to investigate Democrats.

It alleged severe episodes of “criminal” conduct by the president including bribery — rebutting the Republican argument that Democrats have identified no specific criminal wrongdoing by Trump.

“President Trump’s abuse of power encompassed both the constitutional offense of ‘bribery’ and multiple federal crimes,” the 658-page report said, adding Trump’s conduct was “unlike anything this nation has ever seen.”

The House Rules Committee was set to meet Tuesday to lay down guidelines for floor debate on impeachment.

When the Democratic-controlled House of Representatives convenes Wednesday to weigh the two charges, Trump is expected to become only the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998.

Richard Nixon resigned in 1974 just before a House impeachment vote. Neither Johnson nor Clinton was convicted in the Senate.

Trump is also unlikely to be removed from office by the upper house Senate, where Republicans hold a 53-47 majority.

But Schumer has pressed for a fair process, writing Senate Majority Leader Mitch McConnell to demand four key witnesses testify, including acting White House chief of staff Mick Mulvaney and Trump’s ex-national security advisor John Bolton.

“To engage in a trial without the facts coming out is to engage in a cover-up,” he told reporters.

Schumer also sought to set limits on testimony length and questioning of witnesses.

“Just the facts. We don’t need fishing expeditions,” Schumer told CNN. “We’re trying to have the kind of justice America is known for, which is swift but fair justice.”

The White House swatted away Schumer’s “laughable” letter to McConnell.

Senate rules on impeachment are determined by a simple majority vote in the chamber. Although Schumer is looking to strike a deal with McConnell on the rules, it will be the will of the majority that wins out.

Democrats have bridled at McConnell’s recent promise of “total coordination” with the White House, as well as Senator Lindsey Graham’s apparent dismissal of the need to be an impartial juror in the process.

“I’m not trying to pretend to be a fair juror here,” Graham said Sunday, rejecting the charges against Trump as “partisan nonsense.”

One of the two impeachment articles to go before the House charges Trump with abuse of power for conditioning military aid and a White House meeting on Ukraine’s investigating Democrats ahead of the 2020 presidential election.

The other charges him with obstructing Congress for refusing to cooperate with the inquiry and ordering other officials not to appear.

“The Impeachment Hoax is the greatest con job in the history of American politics!” Trump boomed Monday on Twitter.

Democrats have long feared moderate members of the party from Trump-friendly districts could lose their seats next year if they vote to impeach.

One Democrat opposed to impeachment, New Jersey’s Jeff Van Drew, is expected to switch allegiance to the Republican Party this week.

A handful of freshman Democrats are mulling opposing impeachment.

But two of them, including Utah moderate Ben McAdams, announced Monday that they would vote to impeach Trump, a boost to House Speaker Nancy Pelosi’s effort to present a united front in her chamber’s vote.

A third freshman moderate, Abigail Spanberger, was reportedly going to announce later Monday that she, too, has committed to voting to impeach.

Trump was buoyed meanwhile by a new Quinnipiac University poll that showed him matching his best job approval rating ever, with 43 percent of registered voters supporting and 52 percent opposing his job performance.

Shortly before the impeachment hearings began in October, Trump’s job approval stood at just 38 percent.

The White House has been pushing the strong economy, with the lowest unemployment rate in decades and record high stock market numbers.