Republican and Democratic senators are gearing up for an intense battle over witnesses at an impeachment trial likely to set the tone for the 2020 elections.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellHawley warns Schumer to steer clear of Catholic-based criticisms of Barrett Senate GOP set to vote on Trump's Supreme Court pick before election Harris slams Trump's Supreme Court pick as an attempt to 'destroy the Affordable Care Act' MORE (R-Ky.) and Minority Leader Charles Schumer Chuck SchumerPelosi: Trump Supreme Court pick 'threatens' Affordable Care Act Will Democrats attempt to pack the Supreme Court again? Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' MORE (D-N.Y.) have yet to start negotiating on the parameters for the trial, but lawmakers are already jockeying over key questions such as how long a trial should last, whether witnesses will be called and if the White House will be subpoenaed for documents.

Democrats and Republicans appear to be on the cusp of trading rhetorical places.

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House Republicans repeatedly decried the impeachment process as a sham as the House Judiciary Committee debated articles of impeachment on Thursday.

Once impeachment moves to the Senate, Democrats might borrow that line to accuse McConnell and the GOP of running a sham trial if they move to hold it speedily and without key witnesses.

McConnell and Senate Republicans are rallying around the idea of holding a relatively short trial that would consist of a presentation of the two articles of impeachment by House prosecutors and President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE’s legal defense.

At that point, McConnell wants a vote on whether the articles of impeachment should come up for a final up-or-down vote. If that vote failed, the trial would continue and witnesses and additional documentation would be called for or subpoenaed.

“You get to a point where the House has made its case, the White House has made its case and Mitch is on the phone and says, ‘Mr. President, we can end this today or we can drag this out for another month. The votes are there today for you,’” said one Republican senator, who predicted that the president and most senators will be ready to end the trial and vote on the impeachment articles before witnesses are called.

“I don’t want any witnesses and most of us don’t want any witnesses,” the senator added.

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Democrats warn that Republicans will pay a political price if they cut the trial short. They want to hear from acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE, former national security adviser John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE and possibly acting Office of Management and Budget Director Russ Vought.

“Those are obvious witnesses,” said Senate Democratic Whip Dick Durbin Richard (Dick) Joseph DurbinHawley warns Schumer to steer clear of Catholic-based criticisms of Barrett Two Judiciary Democrats say they will not meet with Trump's Supreme Court pick Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election MORE (Ill.). “Whether it’s testifying in person or by deposition, that’s what we went through in the Clinton impeachment. It’s reasonable if this is truly going to be a trial.”

In 1999, House Republican impeachment managers played a video in the well of the Senate floor of White House intern Monica Lewinsky testifying about her affair with President Clinton.

Democrats say Republicans are pushing a double standard if they deny witness testimony.

“They’re running a risk here they ought to think about. If they don’t take this seriously, I think it will not be to their benefit. Even if they are going to vote with the president, regardless, they have to show some respect to the Constitution and due process,” Durbin added.

Sen. Amy Klobuchar Amy Klobuchar3 reasons why Biden is misreading the politics of court packing Social media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight MORE (D-Minn.), who hosted a Democratic Steering Committee meeting Wednesday where impeachment strategy was discussed, said, “I think we should hear from witnesses and I would most like to hear from all the president’s men.”

Republicans warn that if Democrats press for Mulvaney, Bolton and Pompeo to testify, they will demand former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE and his son Hunter testify about their possible ties to Ukrainian corruption.

“That’s a joke. You start that and you’ll have World War III. We’ll call in Joe Biden, we’ll call in Hunter Biden, we’ll call in a lot of people,” said one GOP senator.

Chief Justice John Roberts will preside over a trial, and Vice President Pence will not be able to cast tie-breaking votes. If there is a tie, the motion offered will fail.

All of this puts a handful of senators in both parties under an even bigger spotlight.

The GOP holds 53 seats and can only afford three defections, assuming the 47 Democrats can stay unified.

The swing GOP votes are seen as Sens. Susan Collins Susan Margaret CollinsSenate GOP set to vote on Trump's Supreme Court pick before election Democratic senator to party: 'A little message discipline wouldn't kill us' Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy MORE (Maine), Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (Utah) and Lisa Murkowski Lisa Ann MurkowskiSenate GOP set to vote on Trump's Supreme Court pick before election Supreme Court fight pushes Senate toward brink Democratic senator to party: 'A little message discipline wouldn't kill us' MORE (Alaska), possibly along with Sen. Cory Gardner Cory Scott GardnerBreaking the Chinese space addiction Trump dumbfounds GOP with latest unforced error Billionaire who donated to Trump in 2016 donates to Biden MORE (Colo.), who faces a tough reelection race next year.

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The swing Democrats are seen as Sens. Joe Manchin Joseph (Joe) ManchinTrump taps Amy Coney Barrett for Supreme Court, setting up confirmation sprint Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election Supreme Court fight pushes Senate toward brink MORE (W.Va.) and Doug Jones (Ala.), whose home states voted overwhelmingly for Trump in 2016, and Sen. Kyrsten Sinema (Ariz.), a maverick who largely shuns the press.

Collins told The Hill that she will make a decision on witnesses after she sees what procedural agreement McConnell and Schumer reach but pointed to the 1999 Clinton trial as an important precedent.

“The two leaders have yet to meet to come up with a set of rules. In the last trial the agreement was there would not be witnesses who testified in person, but the Senate did order three depositions,” she said. “I want to wait to see what Sen. McConnell and Sen. Schumer come up with.”

Murkowski predicted the topic of witnesses would be intensely discussed over the next few days.

“There’s going to be a lot of discussion back and forth on that,” she said.

Gardner did not rule out the possibility of bringing in additional witnesses.

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“We’re going to have a process that’s fair and thorough over here, unlike the House circus and partisan sideshow that they ran,” Gardner said, referring to House impeachment hearings.

Another GOP senator who requested anonymity expressed open-mindedness about calling additional witnesses.

“Generally, I’m always on the side of give me all the information you can provide. I’m happy to hear any and all sides,” the lawmaker said, adding that GOP leaders had not yet been in touch to press for a specific outcome.

McConnell indicated Tuesday that he may begin talks with Schumer on the trial rules as soon as next week.

“We’ll be talking about the way to go forward and see if we can reach an agreement,” he said.

McConnell last week told reporters that if he and Schumer fail to agree on a resolution setting the trial rules, he’ll try to muster 51 votes within his own conference to allocate time to each side and decide whether special provisions should be made for additional witnesses and documents.

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The GOP leader predicted earlier this week that these procedural questions would be voted on by the entire Senate instead of being decided by Roberts.

“The way this will work, I would anticipate the chief justice would not actually make any rulings. He would simply submit motions to the body and we would vote,” McConnell told reporters Tuesday.

“Here’s what I would anticipate. The House managers would come over [and] make their arguments. The president’s lawyers would then respond. And at that point the Senate has two choices: It could go down the path of calling witnesses and basically having another trial, or it could decide — and again, 51 members could make that decision — that they’ve heard enough,” he said.

A senior Republican senator said Republicans will simply vote up or down on the articles of impeachment and not any special resolution that would declare Trump acquitted.

A GOP official familiar with the Senate’s impeachment rules and behind-the-scenes discussion said that any rules package or motion related to the trial should be decided while Roberts is in the chair presiding.