WASHINGTON – After Donald Trump was caught on tape talking sexually about women, on Saturday he faced a growing challenge to make sure his GOP presidential candidacy is viable as he heads into the Sunday debate with Hillary Clinton.

Trump is in historically, profoundly, uncharted waters.

The reality show star needs a miracle during the second presidential debate at Washington University in St. Louis – now guaranteed a mega audience – to put his bid back on course.

If Trump goes after Bill Clinton’s sexual lapses at the debate and accuses Hillary Clinton of being an enabler — he’s threatened to do both — Trump risks a giant backlash with the very female voters whose support he needs.

There were a stunning series of developments on Saturday, the day after the bombshell tape surfaced:

• Trump was defiant and defensive on Twitter as top figures in the GOP establishment either called on him to drop out or yanked their support.

“The media and establishment want me out of the race so badly — I WILL NEVER DROP OUT OF THE RACE, WILL NEVER LET MY SUPPORTERS DOWN!” Trump tweeted.

• Some Republicans were exploring what it would take to dump Trump as the nominee.

Former Republican National Committee counsel Ben Ginsberg said on MSNBC that, as a practical matter, such a move would be difficult since early voting has already started in a number of states.

There are provisions to deal with a candidate who drops out. Under Rule 9, the RNC is “authorized and empowered to fill any and all vacancies.” But that’s not the same as charting a legal path to boot Trump off the ticket.

• Melania Trump, usually the silent partner who rarely is seen on her own, rebuked her husband’s crude comments in a brief statement.

“The words my husband used are unacceptable and offensive to me,” she said.

“This does not represent the man that I know. He has the heart and mind of a leader. I hope people will accept his apology, as I have, and focus on the important issues facing our nation and the world.”

• The absence of  a statement from daughter Ivanka, a chief surrogate for her father, was telling.

On Saturday, Trump conversations with radio host Howard Stern surfaced, where he talked about his daughter’s body in graphic terms. “She’s a piece of ass,” Trump told Stern.

• Trump’s running mate, Indiana Gov. Mike Pence, distanced himself from Trump as speculation started on whether he stays on the ticket. Pence said he was “offended” as a “husband and father.”

Pence said Trump needs to use the debate to convince the public of who he really is. Given everything, that may not be the plus Pence thinks it is.

“I do not condone his remarks and cannot defend them,” Pence said in a statement. “We pray for his family and look forward to the opportunity he has to show what is in his heart when he goes before the nation tomorrow night.”

• Vice President Joe Biden said describing Trump’s words as merely “lewd” was not enough. Biden described Trump’s boasting of how he could grab women in their private parts at will as sexual assault. Left unsaid by Biden: sexual assault is a crime.

“The words are demeaning. Such behavior is an abuse of power. It’s not lewd. It’s sexual assault,” Biden said on Twitter.

• Among Republicans calling for Trump to step down on Saturday: Sen. John McCain, former Secretary of State Condi Rice, Sen. Mike Lee, Rep. Jason Chaffetz, Sen. John Thune, Carly Fiorina, Ohio Gov. John Kasich and former California Gov. Arnold Schwarzenegger.

From Illinois, Sen. Mark Kirk and Rep. Rodney Davis over the weekend said Trump should pull out.

The bombshell crude comments were caught on a hot microphone in 2005 while talking to Billy Bush, then with “Access Hollywood.”

The video surfaced after Trump was still recovering from his poor first debate performance, late-night Twitter rants about the weight of a former Miss Universe and revelations that he likely never paid any federal income taxes.

The Sunday debate format is very different from the first at Hofstra University in New York.

It’s a town hall with two moderators, ABC’s Martha Raddatz and CNN’s Anderson Cooper.

Hillary Clinton will come prepared to deal with what ever Trump throws at her about her husband. She knows all about the women at issue.

Trump will have to be very prepared to exploit the hacked emails that surfaced Friday with drafts of Clinton’s highly paid speeches to Wall Street executives.

Trump’s goal Sunday is not to make things worse — a very tall order.