Two Republican groups critical of President Trump are upping the pressure on congressional lawmakers, including Maine Sen. Susan Collins, as the impeachment process heads toward a Senate trial.

A new ad released this week by the Republicans for the Rule of Law wants the Senate to allow witnesses to be called during the trial and urges voters to “Call Sen. Collins and tell her these witnesses must testify.”

Sen. Susan Collins, R-Maine J. Scott Applewhite/Associates Press, file

The ad – which will air in Maine five times each on “Fox and Friends” and “Hannity” over the holidays – follows the formation of a different Republican group that plans to deliver a similar message.

The newly formed Lincoln Project, which opposes Trump’s re-election, last week called on senators to “defend the American Constitution and to put country over partisanship” when it comes to impeachment.

“We will be watching very closely what Sen. Collins and everyone else does,” said Jennifer Horn, a co-founder of the Lincoln Project and former head of the New Hampshire Republican Party. “Where she falls on impeachment will have a significant influence on – I don’t want to say target because I don’t like that word – but it will have an influence on whether she is on the list of Republican senators whose seats we’re looking at.”

Collins, who is running for re-election in what is likely to be the most expensive congressional race in state history, has not said how she will vote on impeachment.

“My hope is that the Senate leaders will negotiate the rules for the trial, just as Republican Senator Trent Lott and Democratic Senator Tom Daschle did for the trial of President Clinton,” she said in a statement Thursday. “Their initial agreement was approved unanimously by all of us serving in the Senate.

“I take seriously the oath I will swear to render ‘impartial justice’ in the impeachment trial. Threats from both the left and the right will have zero influence on my decisions.”

Recent polls show Americans are divided on impeachment, with slightly more people supporting impeachment and Trump’s removal from office.

However, the president’s job-approval rating remains high among Republican voters, most of whom say they are against impeachment. A Gallup poll conducted between Dec. 2 and 15 found 89 percent of Republicans approved of the president’s performance. Only about five percent supported impeachment and removal.

While no GOP senators have said they would vote to remove Trump from office, Sen. Lisa Murkowski, R-Alaska, recently told an Anchorage, Alaska, news affiliate that she is “disturbed” by Senate Majority Leader Mitch McConnell’s plans to hold the impeachment trial in “total coordination” with the White House.

McConnell, of Kentucky, has dismissed the House impeachment and said Senate Republicans will acquit Trump. He has not said whether witnesses will be allowed to testify.

While it would take a two-thirds majority of the Senate to remove Trump from office, the question of whether witnesses will be called and other rules for the trial will be decided by a simple majority. Republicans currently hold 53 Senate seats, so a small number of defections could tip the scales on whether witnesses would be called.

Horn said the Lincoln Project – which also includes George Conway, an attorney, husband of White House adviser Kellyanne Conway and frequent critic of the president – respects Collins’ approach but wants senators to take a stand against Trump in an impeachment trial.

“We have tremendous respect for Sen. Collins,” Horn said. “She has always been an independent voice and has that reputation of looking at each issue separately.”

She added: “I wish more senators would take that approach, especially McConnell. However, the bottom line is we believe there is enough evidence publicly already to support impeachment.”

Horn said she believes many Republicans and right-leaning voters feel the same way. In the two days after the group announced it was forming it raised over $500,000, she said.

At the same time, Republicans for the Rule of Law, a project of the conservative group Defending Democracy Together, this week started running commercials on the Fox News network in Maine to build public pressure on Collins to support calling witnesses during the Senate trial. The Maine ads are part of a $150,000 ad campaign in almost a dozen target states such as Colorado, Pennsylvania and North Carolina where vulnerable Republicans are up for re-election.

The commercial targeting Collins is one of four senator-specific ads planned. The others will focus on Sen. Mitt Romney, R-Utah, Murkowski and Sen. Lamar Alexander, R-Tennessee, who has said he is not seeking re-election.

“After declaring war, impeachment is the most solemn action authorized by the U.S. Constitution and it’s now up to each senator to treat the upcoming trial with seriousness and sobriety, regardless of what party they belong to or what they think of the charges,” Republicans for the Rule of Law spokesman Chris Truax said in a statement Thursday.

“The Senate must hold a fair and open trial – a trial that includes witnesses and procedures designed to establish truth,” Truax said. “Proper trials are seldom comfortable for the accused but that’s no reason not to hold one, even if the accused is President Trump.”