"It's over," said Steve Schmidt, a Republican strategist and senior adviser to the party's 2008 nominee, Senator John McCain. "This is not recoverable. This debate has every likelihood of being a fiasco."
Schmidt predicted the crisis will prompt Trump to bring up the former President's indiscretions, something the billionaire businessman had threatened to do after the first debate and then backed away from.
"It is a certainty that Trump will go down the only path that exists for him, which is try to frame this as a double-standard issue with regard to Bill Clinton," Schmidt said.
Schmidt's old boss, McCain, became the most prominent Republican yet to withdraw support for Trump over the weekend, saying in a statement on Sunday that it had become "impossible to continue to offer even conditional support for his candidacy".
Trump still has his Republican defenders, and on political talk shows they were represented by Rudy Giuliani. The former New York Mayor told ABC that Trump's comments were "reprehensible". But he invoked the Catholic concept of absolution from sin, and sought to draw a line under the controversy: "That was then, and this is now".
Giuliani raised the issue of the Clinton marriage, a possible precursor of arguments that the Republican candidate will use. "I think Donald Trump understands that tonight's debate was always going to be very important, and the stakes have gotten a lot higher," Giuliani said. "I think he'll be up to it."
A survey conducted yesterday by Politico and Morning Consult found that 74 per cent of Republican voters thought the party should continue to back Trump, even after the tape's release.
But pollsters for CBS News, who had been surveying the battleground states of Ohio and Pennsylvania just prior to the scandal, called some of those voters back at the weekend and found that eight in 10 had seen or heard of the video. Almost half said it made them think worse of Trump, although those responders mostly weren't backing the Republican anyway.
Fallout from the 2005 video, which played repeatedly on television all weekend, is likely to overshadow other topics at the debate. After the video's release, CNN also uncovered audio of Trump talking with radio shock-jock Howard Stern in which the two men engaged in lewd conversations about women, including Trump's daughter, Ivanka.
Trump's skills as a showman likely won't be enough to pull him from the muck he's descended into after the recording showed him bragging in obscene terms that his celebrity status allowed him to grope women. The former reality television star vowed yesterday to remain in the race and party leaders appeared to have few options to remove him from the top of the ticket.
Clinton, the first female major party nominee, has held off addressing the controversy until the debate, when she can exploit its potency in front of a larger audience.
Given the weekend drama, viewership could approach the record political audience of more than 84 million for the first debate between Clinton and Trump on September 26. This time, they'll be competing against Hurricane Matthew coverage and a NFL game between the New York Giants and Green Bay Packers.
With less than a month before the election, the timing of the video's release could hardly be worse for Trump and the Republican Party. Ballots are already being cast via early and absentee voting in some states. The impact threatens the party's hold on the Senate and potentially the House.
Even before the latest Trump controversy, polls in recent days have shown Clinton widening her lead nationally, and in key states such as Ohio that Trump probably needs to win, following their first debate.
Trump had been struggling to recover from one of the worst stretches of his campaign following his shaky performance in the first debate, his comments disparaging a beauty pageant winner's weight and personal life, and a New York Times report that he may not have paid any federal taxes for almost two decades following an almost US$1 billion business loss.
The debate will be a town hall-style event, with about half the questions coming from uncommitted voters screened by Gallup, and the rest posed by moderators Martha Raddatz, of ABC News, and Anderson Cooper, of CNN.
After being widely labelled as unprepared for the first debate, Trump practiced taking audience questions at campaign appearances last week, studied videos of the first debate and rehearsed new lines of attack. The town-hall format, one that Clinton has considerably more experience with, will allow the candidates to sit or roam the stage instead of standing behind lecterns.
The format "could be problematic for Trump," said Aaron Kall, director of debate at the University of Michigan. "He ran out of gas toward the end of the first debate, after standing behind a podium for 90 minutes without commercials and breaks."
It may not matter. Schmidt, the Republican strategist, said he can't imagine how Trump can win the election at this point.
"While you have two unpopular candidates for president, there's only one fit candidate for the role of commander in chief," he said.
"Whatever her flaws may be, people will look at her and say that she possesses the requisite qualities of dignity to be a competent and psychological fitness to be the commander in chief."
