Washington(CNN) Once again, President Donald Trump's Fifth Avenue test for fellow Republicans has grown a little harder.

The term refers to Trump's 2016 boast that he could "shoot somebody on Fifth Avenue" without losing support from his political base. For the last five months, revelations about his conduct of Ukraine policy have presented a rolling real-world trial of that proposition.

By the end of last week, Trump stood on the verge of securing his votes of absolution by Senate Republicans who didn't even want to hear new witness testimony. Now, a New York Times report that Trump specified his "quid pro quo" directly to then-national security adviser John Bolton has added a jolt of uncertainty into prospects for a summary acquittal.

Three years of GOP deference to the President suggest Trump will pass eventually whether or not the Senate seeks Bolton's testimony and other new evidence. Nine-in-10 Republican voters approve of his job performance and oppose his removal from office, even as most other Americans do not, according to a recent CNN poll.

And this hardly represents a unique test of GOP acquiescence. Throughout the Ukraine furor, Republican lawmakers have deployed the political equivalent of a bend-but-don't-break defense in football.

Republicans wobbled initially when an intelligence community whistleblower complained Trump had warped US foreign policy at the expense of a vulnerable ally for personal political gain. Sen. Lindsey Graham of South Carolina allowed that evidence of an aid-for-investigations "quid pro quo" would be "very disturbing."

The President's own words -- by telephone with Ukraine's President and directly to journalists after the scandal broke -- corroborated the whistleblower's complaint. So did public statements by his acting White House chief of staff and sworn testimony from several diplomats and national security officials, both career civil servants and Trump appointees.

But by the time House Democrats presented their impeachment case in the Senate trial last week, Republican elected officials had recovered their partisan bearings.

They expressed boredom at three days of painstaking Democratic arguments. After Trump's lawyers offered an understated two-hour opening statement -- concluding that "the President did absolutely nothing wrong" -- Graham pronounced the House case "destroyed."

In reality, the Trump defense so far has nibbled at the edges of Democratic evidence without striking its core. And real-time developments keep posing fresh challenges to the GOP.

After lead House impeachment manager Adam Schiff invoked a CBS News report that Trump had threatened potential defectors with reprisals, Senate Republicans took ostentatious offense. Then Trump buttressed Schiff's point by cautioning the California Democrat that he "hasn't paid a price yet."

Trump's lawyers over the weekend insisted no witnesses had direct evidence that Trump conditioned military aid for Ukraine on announcing investigations of the Biden. Then the New York Times reported, and CNN confirmed, that Bolton's forthcoming book recounts Trump telling him precisely that.

Nothing guarantees that Bolton's testimony would substantially change the calculations of Republican senators. Fox News commentator Brit Hume signaled as much two weeks ago by acknowledging that most Republican senators already believe Trump imposed a quid pro quo even if they won't admit it publicly.

The Trump legal team has prepared a backstop in any event. Even if Democrats prove Trump abused his power, former Harvard professor Alan Dershowitz says, the Constitution does not make that an impeachable offense.

The tribal imperatives of modern Republican politics almost certainly make that enough for Trump to avoid a Senate conviction.

The President's team has played to fears of political, cultural and economic eclipse among the blue-collar white voters the party increasingly depends on. A Senate conviction, White House counsel Pat Cipollone warned senators on Saturday, would "tear up the ballots in all of your states."

Both parties see higher stakes in the trial's effect on voters than on the Senate.

Over the last week, polling averages have shown only a slight uptick in the share of rank-and-file Republicans and independents who favor Trump's removal from office. Trump's approval rating has not fallen, and Republican strategists don't expect it to.

"There are no undecideds here," said GOP pollster Neil Newhouse.

Yet media coverage of the impeachment trial, while still largely confined to cable television, has attracted more public attention than earlier phases of the scandal. That heightens the chance of ripple effects.

"Those who are following it are likely to talk about it," said Democratic pollster Diane Feldman. "It will have at least a residual effect. And with the current level of partisan polarization, residual may be pretty meaningful."

Moreover, the wild-card of Bolton's potential testimony injects a new risk for Republican senators who share the ballot with Trump in November. Even those who ultimately acquit their party's President face their constituents' judgment on whether they reached that decision judiciously.

"The most important thing is not the result," said Democratic pollster Geoff Garin, "but how the voters think they got there."