The Senate is barreling toward a bitter, potentially lengthy fight Tuesday over the rules governing President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE’s impeachment trial.

After two days of pomp and bipartisanship last week, the battle over the trial proceedings is now expected to descend into raucous partisanship that threatens to hang over the rest of the trial.

Democrats immediately castigated the resolution as an attempt to push the debate over impeachment into the early morning hours and out of the public spotlight.

“I think phase one is pretty well determined,” said Sen. Kevin Cramer Kevin John CramerOn Paycheck Protection Program, streamlined forgiveness is key McConnell shores up GOP support for coronavirus package Army Corps urges DOJ to settle case with ND over M DAPL damages MORE (R-N.D.).

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But Democrats say they will force votes on calling additional witnesses and documents as part of the chamber’s debate on the trial rules. The resolution can be amended, giving Democrats an opening to shoehorn in what they view as politically tough votes for GOP senators up for reelection in 2020.

“As soon as Senator McConnell offers this resolution, I will be offering amendments to address the many flaws in this deeply unfair proposal and to subpoena the witnesses and documents we have requested," Schumer said.

Sen. Dick Durbin Richard (Dick) Joseph DurbinMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Senate Republicans signal openness to working with Biden Top GOP senator calls for Biden to release list of possible Supreme Court picks MORE (D-Ill.) said he would leave the timing to Schumer but added that “there’s no question that we will raise this issue."

Democrats want to call former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE, acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE, Mulvaney's senior adviser Robert Blair and Michael Duffey, the associate director of national security at the Office of Management and Budget.

In addition to witnesses, Schumer said Democrats would seek to compel the White House to turn over three batches of documents related to the administration’s decision to delay almost $400 million in aid to Ukraine.

That funding is at the center of Democrats’ impeachment effort. The Government Accountability Office released a report Thursday saying the Office of Management and Budget withheld the appropriated funds last summer in order to advance the president’s own agenda in violation of the Impoundment Control Act.

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Democrats need four Republican votes to call a witness or compel documents — a step that would, if successful, deal a major blow to Senate Majority Leader Mitch McConnell (R-Ky.) and hand Schumer a significant political victory.

But GOP senators, including potential swing votes, have indicated they will oppose efforts to call additional witnesses before they hear opening arguments and get the chance to ask questions at the trial. Democrats are expected to force another round of votes midtrial, setting up a second pressure point.

Sen. Susan Collins Susan Margaret CollinsSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-Maine) said in a statement she would not vote for witnesses at the outset of the trial, arguing that decision should wait until after the first phase, similar to the 1999 Clinton impeachment proceeding.

“Prior to hearing the statement of the case and the Senators asking questions, I will not support any attempts by either side to subpoena documents or witnesses. Instead, that issue should be addressed at the same point that it was in the 1999 trial,” Collins said.

Sen. Mitt Romney Willard (Mitt) Mitt RomneySenate Republicans face tough decision on replacing Ginsburg McConnell says Trump nominee to replace Ginsburg will get Senate vote GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Utah) has said he wants to hear from Bolton, but told reporters that like Collins he would not support an effort to secure witnesses at the start of the trial.

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“I am not going to vote in favor of witnesses until we’ve heard the opening case,” Romney said.

Sens. Lisa Murkowski Lisa Ann MurkowskiSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (R-Alaska) and Lamar Alexander Andrew (Lamar) Lamar AlexanderChamber of Commerce endorses McSally for reelection Trump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures MORE (R-Tenn.) have said they are keeping an open mind on calling witnesses but have backed McConnell’s strategy on waiting until midtrial to make that decision. The rules resolution is expected to guarantee a vote on whether additional evidence is needed.

“The resolution I am supporting guarantees a vote on whether we need additional evidence at the appropriate time. Except for not including a motion to dismiss after senators have asked questions, the resolution establishes fundamentally the same rules that the Senate approved ... for the Clinton impeachment trial in 1999," Alexander said on Monday.

Despite the prebaked nature of the floor fight, it could still stretch on for hours. The Senate isn’t expected to start the impeachment trial until 1 p.m. on Tuesday. Each amendment to the impeachment trial rules, according to aides, will receive up to two hours of debate, evenly divided between Trump’s legal team and the House managers.

Another curveball is whether senators go into closed session so they can debate the rules, which would involve turning off the cameras and removing staff from the chamber. The closed-door proceeding would be the only way senators could talk on the floor about the rules.

GOP leadership aides have warned that a closed session is possible and any senator could make a motion to cloister their colleagues and try to hash things out privately. But some senators have publicly vowed to try to keep as much of the trial deliberations as open as possible.

“I would like it to be as open — I mean, might there be an occasional moment where we think we might come together in closed session? Yes, but the strong presumption is to be open, and I want to see as much of it open as possible,” Schumer said.

Sen. Thom Tillis Thomas (Thom) Roland TillisWhat Senate Republicans have said about election-year Supreme Court vacancies Chamber of Commerce endorses McSally for reelection Airline job cuts loom in battleground states MORE (R-N.C.) also predicted senators would likely delay going into closed session until after phase one of the trial — opening arguments and questions from senators.

“I doubt it,” he said. “I don’t think we’ll probably go into closed session until after the rules.”