President Donald Trump speaks during news conference with Finnish President Sauli Niinisto in the East Room of the White House, Wednesday, Oct. 2, 2019, in Washington.

So did the president's backing from Republican elected officials in Washington. Except for Sen. Mitt Romney of Utah and a few others, GOP members of the House and Senate have either defended the president's actions concerning Ukraine and former Vice President Joe Biden or remained silent.

Modern polling usually tells us how public opinion hasn't moved. Despite last week's hailstorm of bad news, President Donald Trump's job approval rating stood virtually unchanged.

Yet surveys in recent days show public sentiment evolving more than Trump's steady topline would suggest. They also illuminate the potential for erosion among fellow Republicans, which could ultimately threaten the president's ability to survive a Senate trial on articles of impeachment approved by the House.

"If you're the president you have to take that seriously," says former Republican strategist Tom Davis, who once ran his party's House campaign arm. "What moves this ultimately is public opinion. These members like their jobs."

Last week's Monmouth University poll showed signs of movement within a broader portrait of stability. Buoyed by backing from 86% of Republicans, Trump's approval rating remained unchanged: 41% of Americans approved of his job performance, 53% disapproved. At the same time, the share of Republicans backing a House impeachment inquiry doubled to 16% from 8% in August.

A CBS News poll found 23% of Republicans backing an impeachment probe. In a USA Today survey, 30% of Republicans called it "an abuse of power" for Trump to ask Ukraine to investigate Biden.

Even if they haven't broken with their party's president, those Republicans pose a particular danger to Trump, who once bragged that he could shoot someone on Fifth Avenue in New York without losing support.

"The willingness to hear this out is a sign that you're not a Fifth Avenue Republican," says GOP strategist Liam Donovan.

Others susceptible to change include the roughly 15% of voters who already disapprove of Trump but don't yet back impeachment. This group consists largely of political independents, with some Democrats and a smaller number of Republicans as well.

College-educated whites — who disdain Trump but so far feel less strongly about impeaching him — represent a special vulnerability. If House Democratic investigators can persuade wavering Republicans that Trump withheld aid to Ukraine in return for a Biden investigation, observes GOP pollster Whit Ayres, "then it's a different ballgame."

Tribal loyalty in contemporary politics makes that a high hurdle. In the Monmouth Poll, for example, only 40% of Republicans said they believed Trump requested a Biden investigation even though the White House-released transcript of his call with Ukraine's president shows that he did.

"There are a lot of people who will go to the ends of the Earth to give Donald Trump the benefit of the doubt," Ayres explains.