THE TIGHTENING NOOSE:

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."

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