As the impeachment spotlight shifts to the Senate, several senators are emerging as early pivotal players as Washington gears up for President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE’s trial.

The rules, where a simple majority can make or break decisions, could throw a curveball into the looming procedural fights that will determine what the trial looks like. Democrats will need four GOP senators to successfully call a witness or request documents, and only three GOP senators to block a Republican motion.

A handful of senators in both parties will be under the microscope during the proceeding as the press — and leadership — look for areas where members might break ranks.

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Here are 10 senators to watch:

Mitch McConnell Addison (Mitch) Mitchell McConnellSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE

The Senate majority leader might not have “ball control,” in his words, during the likely weeks-long impeachment trial, but if he keeps most of his caucus united he’ll be able to set the terms of the proceeding.

McConnell (R-Ky.), who is up for reelection in 2020, has tied himself closely to Trump as he plots the GOP strategy and has pledged “total coordination” with the White House — a promise that earned him bipartisan flack.

He’s also been able to exert influence on the White House, winning over officials to his view for a shorter impeachment trial with few, if any, witnesses. He’ll need 51 of his 53 members to successfully muscle through his vision.

Chuck Schumer Chuck SchumerDemocrats scramble on COVID-19 relief amid division, Trump surprise Pelosi, Schumer 'encouraged' by Trump call for bigger coronavirus relief package Schumer, Sanders call for Senate panel to address election security MORE

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Senate Minority Leader Charles Schumer Chuck SchumerDemocrats scramble on COVID-19 relief amid division, Trump surprise Pelosi, Schumer 'encouraged' by Trump call for bigger coronavirus relief package Schumer, Sanders call for Senate panel to address election security MORE (D-N.Y.) is working to gain leverage on Republicans to reach an agreement on the rules of the trial.

Though Trump is unlikely to be convicted in the GOP-controlled Senate, which would require 67 votes, Democrats are hoping to force McConnell — or, absent that, at least four GOP senators — to cut a deal on Ukraine-related documents and witnesses including former national security adviser John Bolton John BoltonThe 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 Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and 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.

Schumer’s strategy has focused on keeping his caucus united on the procedural fight while launching a blitz to build public support. Absent a deal, he’s warned they’ll force votes during the trial.

Lamar Alexander Andrew (Lamar) Lamar AlexanderTrump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures CDC says asymptomatic people don't need testing, draws criticism from experts MORE

Alexander, an ally to McConnell, has emerged as a senator to watch as the chamber crafts the rules of the impeachment trial.

The Tennessee Republican’s decision to retire at the end of 2020 coupled with splits from Trump on high-profile votes, including opposing the emergency declaration on the border wall, have sparked speculation that he could be willing to break rank and support calling witnesses.

Alexander, viewed as a bipartisan dealmaker, has been tight lipped except to say that he hopes McConell and Schumer can get a deal and that he wants it to be a “fair trial.”

Susan Collins

Collins is viewed as a crucial Republican swing vote on both procedure and convicting or acquitting Trump on the two articles of impeachment.

The Maine senator is one of a dwindling group of Senate moderates and one of two GOP senators up for reelection in a state won in 2016 by Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE.

She said this week that she is “open” to calling witnesses but that a specific decision on who, if anyone, needs to testify should wait until after both sides have presented their arguments and senators have asked questions.

She also knocked both McConnell and Democrats for taking a position on the outcome of the trial, saying that it was "inappropriate ... for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us.”

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Doug Jones

Jones is viewed as a potential swing vote in the Democratic caucus because of the Alabama senator’s status as the most vulnerable Senate incumbent on the ballot next year.

Jones has called for a “a full, fair, and complete trial” including additional witnesses and documents. But he’s stopped short of saying how he will vote at the end of the Senate proceeding.

“If those dots aren't connected and there are other explanations that I think are consistent with innocence, I will go that way too,” he told ABC News.

Joe Manchin Joseph (Joe) ManchinSenate leaders quash talk of rank-and-file COVID-19 deal OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency MORE

The West Virginia senator is another Democrat to watch during the votes on whether to convict or acquit Trump.

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Manchin, who won reelection in 2016, votes with Trump more than any other Democratic senator currently in office, and Trump is popular in his state. Manchin was the only Democrat to vote for Justice Brett Kavanaugh Brett Michael KavanaughGideon leads Collins by 12 points in Maine Senate race: poll Conservatives see glaring omission on Trump's Supreme Court shortlist Cruz says he wouldn't accept Supreme Court nomination MORE’s Supreme Court nomination last year.

Manchin has thrown his support behind calling witnesses but characterized himself as undecided when it comes to supporting impeachment.

Lisa Murkowski Lisa Ann MurkowskiGOP ramps up attacks on Democrats over talk of nixing filibuster OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House Democrats tee up vote on climate-focused energy bill next week MORE

Murkowski has emerged as a leading moderate during the Trump administration, casting votes against ObamaCare repeal and Kavanaugh’s confirmation.

The Alaska senator, who once lost a primary and saw her party battle to unseat her in a general election, has declined to tip her hand on impeachment beyond urging McConnell and Schumer to get a deal on the trial proceedings.

She captured headlines over the recess when she told an Alaska TV station that she was “disturbed” by McConnell’s pledge to coordinate with the White House on impeachment.

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But she was also critical of the House process, arguing that if House Democrats wanted to hear from Mulvaney and Bolton, who both defied subpoenas to appear, “then that next step is to go to the courts.” House Democrats warned at the time that the White House could drag out the court battle for months.

Rand Paul Randal (Rand) Howard PaulGOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill Overnight Health Care: Senate Democrats block GOP relief bill | Democrats reveal Medicaid chief's spending on high-paid consultants | Trump calls question about why he 'lied' about COVID-19 a 'disgrace' MORE

Paul (R-Ky.) has emerged as a vocal defender of the president when it comes to impeachment, but he warned in November that he could vote against a rules resolution if it limits witnesses and blocks the whistleblower from being called to testify.

He separately told reporters that he could force a vote to call Hunter Biden, former Vice President Joe Biden Joe BidenThe Memo: Warning signs flash for Trump on debates Senate Republicans signal openness to working with Biden National postal mail handlers union endorses Biden MORE’s son, to testify and floated quickly dismissing the articles of impeachment, a motion he could try to make from the floor during the trial.

The two actions run counter to a growing consensus within the Senate Republican Conference that there should be a speedy trial with no witnesses that ends in Trump’s formal acquittal.

Paul sidestepped when asked by The Hill last month if he would still push for a dismissal, saying that the “whole idea of the impeachment inquiry was ill-conceived ... so I think the quicker it can be done the better."

Mitt Romney Willard (Mitt) Mitt RomneyThe Memo: Warning signs flash for Trump on debates GOP votes to authorize subpoenas, depositions in Obama-era probe Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE

Romney, the Republican Party’s 2012 presidential nominee, has been critical of Trump at times in the lead-up to and during the House’s impeachment inquiry. He called Trump’s suggestion that Ukraine or China investigate the Bidens a “brazen and unprecedented appeal” that was “wrong and appalling.”

But, like Collins and Murkowski, the Utah Republican been tight-lipped and pledged to keep a “completely open mind” during the Senate trial.

He’s also sidestepped saying if he wants to hear from specific witnesses or if he would support Schumer’s request for live testimony. He told reporters before the break that he was "indifferent" about whether the Senate passes one resolution governing both process and witnesses or handles them in separate resolutions.

Kyrsten Sinema

Sinema (D-Ariz.), who joined the Senate in 2019, has largely managed to avoid the national spotlight as Washington tries to game out the impeachment trial.

But the freshman senator quickly emerged as one of the Democratic caucus’s most moderate votes, including voting with Trump nearly 53 percent of the time, according to FiveThirtyEight. That puts her behind only Manchin among sitting Democratic senators when it comes to voting with Trump.

Sinema is known around the Capitol for being tight-lipped and does not do hallway interviews. But she has indicated that she has not made a decision on final votes, telling KNXV, a TV station in Arizona, that she will go into the trial unbiased.

“As our juror it will be my constitutional duty to approach it with no bias,” she said. “And to listen to the arguments presented by both sides.”