Four Republicans are needed to break party lines and support calling witnesses. Here’s who may be on the fence:
Mitt Romney, Utah; Lisa Murkowski, Alaska; Lamar Alexander, Tennessee; Susan Collins, Maine
On Monday, Mitt Romney, the Beehive State’s junior senator, said it is “increasingly likely” enough Republican senators would join with Democrats to support calling Bolton as a witness.
“It’s important to be able to hear from John Bolton for us to be able to make an impartial judgment,” Romney said.
That prompted a swift rebuke from the most junior senator of all, Georgia's Kelly Loeffler.
Lisa Murkowski, Romney and Lamar Alexander are senators being targeted to allow witnesses by two anti-Trump GOP groups. Republicans for the Rule of Law and the Lincoln Project both oppose Trump's reelection.
Susan Collins, who was also targeted by the groups in a series of ads earlier this month, has been on record since early January as being open-minded about the entire process of calling witnesses.
»MORE: Two anti-Trump groups upping pressure on Senate Republicans
“From the outset, I have said that we should follow the model that we used with the [Bill] Clinton impeachment trial,” said Collins, who also served as a senator during Clinton’s trial. “At the conclusion of that phase of the 1999 trial, the Senate voted on a motion to subpoena witnesses and admit additional materials after the case had been heard and the questions had been posed.
“I voted in favor of that motion subpoenaing witnesses.”
President Bill Clinton was impeached on perjury and obstruction of Congress charges on Dec. 19, 1998. He was acquitted by the Senate on Feb. 12, 1999. Fourteen senators from that trial still remain in office. Here's how they voted. Susan Collins, R-Maine. Not guilty on both counts Michael D. Crapo, R-Idaho. Guilty on both counts Richard Durbin, D-Illinois. Not guilty on both counts Mike Enzi, R-Wyoming. Guilty on both counts Dianne Feinstein, D-California. Not guilty on both counts Charles Grassley, R-Iow
Collins said in Trump’s trial, like Clinton’s, “both sides should have the opportunity to state their case and the senators should have the opportunity to pose questions. Then, the Senate should have an up-or-down-vote on whether to subpoena witnesses and documents.”
»MORE: Trump’s lawyers wrap up defense | What comes next
Collins believes “having additional information would be helpful. It is likely that I would support a motion to call witnesses at that point in the trial just as I did in 1999.”
Pat Toomey, Pennsylvania
According to The Washington Post, Pat Toomey has expressed openness to a witness agreement in which Democrats could call one witness and Republicans could call their own witness.
Rob Portman, Ohio
Rob Portman has said Trump's July 25 call with Ukrainian President Volodymyr Zelensky was far from "perfect," according to The Columbus Dispatch. That call is at the very heart of Democrats' impeachment claims.
The president is accused of abusing his office by asking Ukraine to investigate former Vice President Joe Biden and his son, Hunter, while allegedly withholding aid from Ukraine, a U.S. ally at war with bordering Russia. The second article of impeachment accuses him of obstructing Congress by refusing to turn over documents or allow officials to testify in the House probe.
Jerry Moran, Kansas
While some Republicans found House impeachment managers' arguments against the president dull and lifeless, Jerry Moran wasn't among them, merely saying they were "repetitive," according to The Washington Post. Moran, according to CNN, is among senators being watched as a possible swing vote.
Cory Gardner, Colorado
Cory Gardner defeated Democrat Mark Udall in 2014 and is up for reelection this year. But the Rocky Mountain state has a large number of unaffiliated voters, according to CPR, and a Democratic blue wave swept through Colorado in the 2018 midterms, as it did in much of the nation. Gardner is walking a fine line and finds himself in a tough spot.
»MORE: Which U.S. presidents have faced impeachment?
The U.S. Constitution requires a two-thirds majority in the Senate, or 67 senators, to convict in an impeachment trial.
Republicans hold 53 seats in the Senate, while Democrats hold 45. However, two Independents — including presidential candidate Bernie Sanders of Vermont — regularly caucus with Democrats, giving the nation’s blue party 47 votes.
If the Senate votes along party lines regarding impeachment — as did the House — 20 Republican senators would have to join Democrats in convicting Trump and removing him from office.
Here are the key figures in President Donald Trump's impeachment trial. House Speaker Nancy Pelosi. She led the impeachment effort. Chief Justice John Roberts. He will preside over the trial. The Senate's political leaders - Majority Leader Mitch McConnell and Minority Leader Chuck Schumer. The president's legal defense team - White House counsel Pat Cipollone, Trump personal lawyer Jay Sekulow, Kenneth W. Starr, Alan Dershowitz, along with Robert Ray and Jane Raskin. House Democratic impeachment managers
The first article of impeachment passed by the House charges Trump with abuse of power.
Democrats allege Trump “solicited the interference of a foreign government, Ukraine, in the 2020 U.S. presidential election. He did so through a scheme or course of conduct that included soliciting the government of Ukraine to publicly announce investigations that would benefit his reelection, harm the election prospects of a political opponent, and influence the 2020 U.S. president election to his advantage.”
»Read the best lines from President Trump’s Senate impeachment trial
The “election prospects of a political opponent” refer to Biden, currently a front-runner in a narrowing field of Democratic White House hopefuls.
The president “also sought to pressure the government of Ukraine to take these steps by conditioning official U.S. government acts of significant value to Ukraine on its public announcement of investigations.”
Democrats argue the president “used the powers of his presidency in a manner that compromised the national security of the United States and undermined the integrity of the United States democratic process. He thus ignored and injured the interests of the nation.”
How is your senator likely to vote on impeachment?