Trump is also set to appear next week at a reception for Senate Leadership Fund, a super PAC closely aligned with Majority Leader Mitch McConnell and party leadership.

The offensive comes as Trump presses Republicans to remain united behind him. During a cabinet meeting last week, the president implored Republicans to “get tougher and fight” while lamenting that Democrats “stick together. You never see them break off.”

The president is looking to buck up senators coming under mounting pressure on impeachment. Gardner, who is widely seen as the most jeopardized Republican incumbent up for reelection, faced criticism earlier this month after he dodged questions about Trump’s conduct. Ernst was confronted at a town hall over her support for the president. This week, a liberal group began a multimillion-dollar advertising campaign pushing both Republican senators to support impeachment.

The new online fundraising drive bypassed Collins, an occasional Trump critic who called on the president to retract his tweet comparing the impeachment investigation to a “lynching.” Collins also said Trump made a “big mistake” in asking China to investigate the Biden family.

The Maine senator has avoided taking a position on impeachment because, she says, as a juror in a prospective Senate trial she doesn't want to "prejudge" the proceedings.

Arizona Sen. Martha McSally, another vulnerable Republican facing reelection, was also omitted, though apparently for a different reason. While McSally signed onto the anti-impeachment resolution, she has frustrated Republican officials over her reluctance to exclusively use WinRed, a Trump-endorsed online fundraising tool. Party officials are trying to turn WinRed into a centralized hub of small-donor giving ahead of the 2020 election and used the platform to send out Trump’s appeal for the three senators.

Collins and McSally are missing out on a potential windfall after they were both outraised by their Democratic rivals during the third quarter of the year. McSally's Democratic opponent, former astronaut Mark Kelly, has raked in $5 million more than her over the course of the year.

McSally's campaign declined to comment, and a spokesperson for Collins did not respond.