Washington (CNN) Even before Donald Trump is impeached, the partisanship of his looming Senate trial is casting doubt on whether polarized Washington can hold a President to account -- now and in the future.

Democrats on Sunday accused Republicans of subverting bedrock values on which America is founded after Republican Senate leadership abandoned any notion that they had not pre-judged the case.

Furious exchanges came three days before the Democratic-led House is expected to vote to saddle Trump with the historic shame of impeachment over his demand for favors from Ukraine.

The bitterness underscored how the saga has deepened divisions highlighted by Trump and how his power over the GOP is causing party lawmakers to ignore or deny the grave charges against him.

The President, meanwhile, fulminated on Twitter all weekend, inciting fresh political uproar to intimidate vulnerable House Democrats and to electrify the base he needs for reelection.

But there were also signs that the last few weeks have inflicted lasting political damage on the commander in chief as a new Fox News poll showed fully half the American voters back impeachment.

Trump is facing two articles of impeachment -- abuse of power and obstruction of Congress -- over his pressure on Ukraine, including for a probe into one of his Democratic 2020 challengers, Joe Biden.

The House rules committee will meet Tuesday to discuss next steps for a full House vote to make Trump the third impeached President as early as Wednesday.

An already fevered atmosphere over impeachment deteriorated further during the weekend after Senate Majority Leader Mitch McConnell said he was working closely with the White House counsel to prepare for a Senate trial next year.

Senate Minority Leader Chuck Schumer made it clear in a letter to McConnell Sunday night that he prefers a Senate impeachment trial with witness testimony and new documents.

In the letter obtained by CNN , Schumer, a New York Democrat, called for at least four witnesses to testify, including acting White House chief of staff Mick Mulvaney, former national security adviser John Bolton, senior adviser to the acting White House chief of staff Robert Blair and Office of Management and Budget official Michael Duffey.

McConnell spokesman David Popp said Sunday night that "Leader McConnell has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon. That timeline has not changed."

House Judiciary Chairman Jerry Nadler accused Republicans earlier Sunday of ignoring their duties under the Constitution in prejudging the evidence in an already all-but-certain push to acquit Trump.

"(Senators) have to pledge to do impartial justice. And here you have the majority (leader) of the Senate, in effect the foreman of the jury, saying he's going to work hand in glove with the defense attorney," Nadler, a New York Democrat, said on ABC's "This Week." "And that's in violation of the oath that they're about to take, and it's a complete subversion of the constitutional scheme."

'Disdain' for the accusations

On Saturday, Republican Senate Judiciary Chairman Lindsey Graham told CNN's Becky Anderson in Doha that he wasn't pretending to be a "fair juror." He went further on Sunday, saying he wanted to dispense with the Senate trial as quickly as possible.

"I (have) clearly made up my mind. I'm not trying to hide the fact that I have disdain for the accusations and the process. So I don't need any witnesses," Graham, a South Carolina Republican, told CBS' "Face the Nation."

Graham is among a number of Republicans who want to resolve the trial as soon as possible, given the almost certain lack of a two-thirds Senate majority needed to convict Trump.

Republicans have done little to directly repudiate damning evidence from senior officials unearthed by the House impeachment investigation. They have instead accepted Trump's claims that his behavior was "perfect" and now argue that abuse of power is not an impeachable offense since it is not a crime -- even though foreign interference in a US election was perhaps a fear that preoccupied the founders more than any other. They have also criticized the way Democrats ran the impeachment inquiry in the House.

Republican senators seem to want to avoid the possibility that Trump's hopes to put up favorable witnesses in a televised show featuring his pet conspiracy theories could besmirch the dignity of their chamber and backfire politically.

But Democratic House Intelligence Committee Chairman Adam Schiff , who called Trump a "clear and present danger to democracy," argued that the Senate should call witnesses in the trial in order to properly consider the case made on the other side of Capitol Hill.

"I think there are any number of witnesses that should be called in a Senate trial, and many witnesses the American people would like to hear from that the administration has refused to make available," Schiff said on "This Week."

"I think we see clearly what's going on here with the comments of Lindsey Graham and others, and that is they don't want the American people to see the facts," Schiff said.

But on CNN's "State of the Union" Republican Sen. Rand Paul of Kentucky argued that Trump was well within his rights to hold back nearly $400 million in US military aid to Ukraine over concerns about corruption in the former Soviet state.

"When the Democrats say, 'oh, we damaged national security' by holding up for 55 days ... money that was going to Ukraine, I say, well, we shouldn't do it in the first place," Paul, a skeptic of most foreign aid, said.

Claims that Trump was primarily concerned about corruption in Ukraine are likely to take center stage in the Senate trial. But their credibility is challenged because the President did not actually mention such wider worries in his July 25 call with Ukrainian President Volodymyr Zelensky

He did specifically ask for an investigation into Biden. And a crackdown on corruption has not been at the center of administration foreign policy in any other nation.

Trump unrepentant

Trump is showing every sign that he will become even more unrestrained after he is impeached.

He met Friday with his personal lawyer Rudy Giuliani -- who is just back from Ukraine on a mission to dig for dirt against Biden -- the exact conduct that helped trigger the impeachment drama in the first place.

And on a weekend Twitter tear, the President picked up on a new Republican line of defense that there was no case to answer because he asked Zelensky to do "us" a favor -- as in the US -- rather than a personal favor for himself.

"A PERFECT phone call. 'Can you do us (not me. Us is referring to our Country) a favor.' Then go on to talk about 'Country' and 'U.S. Attorney General,' " Trump wrote on Twitter. "The Impeachment Hoax is just a continuation of the Witch Hunt which has been going on for 3 years. We will win!"

Trump's claim is however undermined because his request to Zelensky referred to a conspiracy theory that Ukraine and not Russia interfered in the 2020 election.

It is not clear why that favor -- relating to a personal political obsession of Trump -- would be in the national interest. A conventional interpretation of US interests in Ukraine would center on supporting a government under siege from Vladimir Putin's Russia.

Trump also tried to persuade Democratic New Jersey Rep. Jeff Van Drew , who is against impeachment, to switch sides. If Van Drew crosses the floor, he would be greeted as a hero by Republicans. But Democrats will argue he is quitting because his opposition to impeachment has alienated his own party supporters. Five of Van Drew's staffers quit Sunday night, saying they can "no longer in good conscience continue our service," according to a resignation letter obtained by CNN. A sixth also resigned.

It is not a surprise that partisan feelings are shaping the impeachment end game -- after all, by its nature, it is a political process prone to shaping by public opinion.

And Democrats have offered Republicans an opening by declining to challenge Trump's refusal to provide key witnesses and testimony under a claim of absolute executive privilege in the courts, reasoning it would take too long.

Yet the partisan approach of the GOP underscores the fact that for most Republicans, even admitting any kind of wrongdoing by the President would be akin to political suicide.

That's why this impeachment drama -- unlike Bill Clinton's 20 years ago -- has not involved calls for an apology from the President for his own side or any discussion about whether he has transgressed, albeit in a way that does not merit impeachment.

The absence of such discussion is one reason why this impeachment showdown could have a long legacy as it will effectively enshrine a precedent of a President using executive power to lean on a foreign country for his personal political gain.

And the Republican chorus that Trump did nothing wrong and a refusal to even examine an impeachment case suggests a blueprint for future scandals. The party of an unrestrained President could henceforth spare him or her from scrutiny as long as they control the Senate and there is no super majority to convict.