David Wasserman, who works for the nonpartisan Cook Political Report, said he has had private conversations with Republican congressmen who believe Mr Trump is “wildly unfit to be president”.
“They can’t say that in public, or else their political careers would be torpedoed by one tweet from the Oval Office,” he told the New York Times.
As evidence mounts showing Mr Trump attempted to pressure Ukraine’s leader to investigate his political rival Joe Biden, some Republicans have tried to keep their distance from the president without explicitly supporting an impeachment inquiry.
Mark Amodei, a Republican representative for Nevada, received backlash from his party when he said Congress needed to “follow the facts” on impeachment.
In a hyper-partisan Congress, Mr Amodei’s comment was viewed as a significant intervention and forced him to explain himself to the Trump campaign’s political director, top House Republicans and the acting White House chief of staff.
“I just think you have to respect the process,” he said.
“I think you need to be transparent, and you need to tell the truth.”
A whistleblower complaint has claimed Mr Trump sought to use military assistance for Ukraine as leverage to push its president, Volodymyr Zelensky, into investigating unsubstantiated corruption allegations against Mr Biden and his son Hunter.
An impeachment inquiry against Mr Trump was initiated last month by Nancy Pelosi, the House speaker, due to the allegations.
Although Democrats have supported the investigation, Republicans have largely dismissed concerns about the president’s conduct.
In a campaign advert released this week, Mitch McConnell, the Republican Senate majority leader, said he would not allow an impeachment inquiry to succeed.
“Nancy Pelosi is in the clutches of a left wing mob,” Mr McConnell said in the advert.
“The way that impeachment stops is a Senate majority with me as majority leader.”
However, some anti-Trump conservatives have been emboldened by the inquiry.
Republicans for the Rule of Law, from the anti-Trump group Defending Democracy Together, is planning to spend more than $1m to run adverts on Fox and MSNBC calling for Republicans to “demand the facts” about Trump and Ukraine.
On Friday, Mitt Romney, the Republican presidential nominee in 2012, denounced Mr Trump on Twitter and said his attempts to get Ukraine to investigate Mr Biden were “wrong and appalling.”
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