NEW YORK • Support for billionaire Donald Trump has plunged across the swing-state map over the last 10 days, wiping out his political recovery from last month and threatening to undo weeks of Republican gains in the battle for control of Congress.
Should he falter badly in his second debate with Democrat Hillary Clinton on Sunday in St Louis, Republican congressional candidates may take it as a cue to flee openly from their nominee, said two senior Republicans involved at high levels of the campaign, who spoke on condition of anonymity because they were not authorised to discuss private party strategy.
For his party, Mr Trump's reversal in fortune comes at the worst possible moment: Having muted their criticism of him in hopes that he could at least run competitively through Election Day, Republicans must decide in the next few days, rather than weeks, whether to seek distance from his wobbly campaign.
Mr Trump has already slipped perceptibly in public polls, trailing widely this week in Pennsylvania and by smaller margins in Florida and North Carolina - three states he cannot afford to lose.
Private polling by both parties shows an even more precipitous drop, especially among independent voters, moderate Republicans and women, according to a dozen strategists from both parties who spoke on condition of anonymity because the data was confidential.
Mr Trump's erratic behaviour last week after his poor performance in the first debate with Mrs Clinton - attacking a former beauty pageant winner over her weight, and making an issue of the Clintons' marriage - has alarmed a number of Republican senators, including Senator Mitch McConnell, the majority leader.
Mr McConnell expressed concern that Mr Trump could lose even more support among women, according to a Republican official who spoke on condition of anonymity to recount a private conversation.
"Two weeks ago, I would have said Republicans would hold control of the Senate, but there's just so many seats up and nobody is getting separation," said Mr Neil Newhouse, a Republican pollster, referring to the number of the party's candidates still locked in tight races.
In contrast, Mr Trump's running mate Mike Pence is basking in the positive reviews from his vice-presidential debate performance opposite Mrs Clinton's pick Tim Kaine.
Mr Trump hailed Mr Pence's steady performance as a credit to his own judgment on Wednesday, in a renewed attempt to allay voters' concerns. "America also got to look first-hand at my judgment - and that was judgment," Mr Trump said in Nevada. "You know you need judgment for people, for deals."
With a swift and not-so-subtle effort to link Mr Pence's debate outing back to the top of the ticket, Trump advisers fanned out on television on Wednesday to make the most of Mr Pence's moment.
While it is doubtful that Mr Pence's performance will do much to move voters towards Mr Trump, the hope among Trump allies is that Mr Pence may have reassured Americans by proxy.