US election

Mr Trump also said Republicans were coming at him "from all sides" and their disloyalty was harder to fight than the Democrats.

Mr Ryan is the latest Republican not to back Mr Trump after a 2005 video emerged showing him making obscene comments about groping women.

However Mr Trump has been strongly supported by running mate Mike Pence.

"You have nominated a man for president who never quits, who never backs down. He is a fighter, he is a winner," Mr Pence said at an event in Iowa, praising him for apologising for his obscene comments during Sunday's debate.

Mr Trump is due to meet his Democratic rival Hillary Clinton in the second debate at 21:00 local time (01:00 GMT).

His adviser Rudy Giuliani said Mr Trump may well bring up Mrs Clinton's alleged role in discrediting women who accused her husband Bill of sexual abuse.

A 2005 recording of Mr Trump reveals him bragging about groping women.

At least 33 senior Republicans - including senators, members of Congress, and state governors - have withdrawn their support since the video surfaced on Friday.

The Republican National Committee (RNC) sets out in its Rule 9 the terms for "filling vacancies in nominations".

It reads: "The Republican National Committee is hereby authorized and empowered to fill any and all vacancies which may occur by reason of death, declination, or otherwise of the Republican candidate for President of the United States."

Death may be off the agenda, so what about the other scenarios?

Declination...

Mr Trump has been under pressure after a tape from 2005 of him making obscene comments and bragging about groping and kissing women emerged on Friday.

He told the Wall Street Journal there was "zero chance I'll quit" and he was getting "unbelievable" support.

Top Republicans have condemned Mr Trump's remarks in the video.

Since the tape emerged, at least 10 Republican senators have either said they will not be voting for the Republican candidate in the general election in 30 days' time, or have called for him to stand aside.

In the video, posted by the Washington Post, Mr Trump is heard telling TV host Billy Bush "you can do anything" to women "when you're a star".

The New York businessman bragged about trying to have sex with a married woman as well as kissing and groping others.

Mr Trump said he held back "because he didn't want to hurt anyone's feelings".

In a Fox News interview, he also accused moderator Lester Holt of being tougher on him than on Mrs Clinton.

Overnight polls with small samples were split on who won, but more rigorous surveys are due in the coming days.

Initial estimates from research firm Nielsen suggest more than 80 million Americans watched the debate at home, but that figure will rise significantly when other forms of viewing are counted.

Republican Donald Trump and Democrat Hillary Clinton will take to the stage in New York on Monday night.

The duel at Hofstra University could be the most watched debate in television history, with 100 million viewers.

There are 43 days until the election, with one poll on Monday suggesting some movement towards Mr Trump.

Mr Trump suggested Mrs Clinton's security detail should give up their guns and "see what happens to her".

He told supporters his rival wanted to "destroy your second amendment" - referring to the right to own guns.

Mrs Clinton's team has accused Mr Trump of "inciting people to violence".

Speaking at a rally in Miami on Friday, the Republican candidate said: "I think that her bodyguards should drop all weapons. They should disarm, right?'

The Republican candidate had been a leader of the "birther" movement that questioned Hawaii-born Mr Obama's citizenship.

But his campaign now accuses his Democratic rival Hillary Clinton of introducing the "smear" during the 2008 Democratic nomination contest.

There is no evidence to link Mrs Clinton to the birthers.

In reaction she tweeted that President Obama's successor "cannot and will not be the man who led the racist birther movement".

Mr Trump called for more troops, more planes and more boats at a rally in Philadelphia.

He also wants US generals to come up with a plan to defeat the self-styled Islamic State (IS) in his first 30 days in the White House.

Recent polls show the race for the presidency has narrowed.

Democrats and even some Republicans have painted Mr Trump as unfit to serve as US commander-in-chief but he has made up some ground on rival Hillary Clinton.