WASHINGTON (Reuters) - Republican presidential candidate Jeb Bush’s campaign has looked into whether it is possible to withdraw a pledge to support the eventual nominee if it is Donald Trump, a senior Bush aide said on Thursday, in a sign of the deep enmity between the two.

U.S. Republican presidential candidate Jeb Bush waits to speak at the Devine Millimet FITN Candidate Series Forum in Manchester, New Hampshire in this December 8, 2015 file photo. Super PACs associated with 2016 Republican U.S. presidential hopefuls Jeb Bush and Chris Christie have led the way in spending thus far, but the TV, radio and Internet ads they have purchased have failed to lift either candidate in opinion polls. REUTERS/Brian Snyder/Files

The development came to light after Bush and Trump engaged in bitter exchanges at the last Republican debate of the year on Tuesday night in Las Vegas.

Last week in New Hampshire, Bush insisted he would not need to withdraw his support for the nominee because he did not believe Trump would win the 2016 Republican presidential nomination.

Behind the scenes, top aides have looked into whether Bush would still qualify to be on the ballots in all 50 states because some states require a loyalty pledge in order to be on the ballot.

A driving factor behind the move was Trump’s proposal to ban all Muslims from entering the United States, a step that Bush denounced to Trump’s face at the debate.

“We received lots of questions following Donald Trump’s most recent unhinged proposal so our campaign did due diligence looking into the rules surrounding the pledge,” said a senior Bush aide who spoke on condition of anonymity.

Still, Bush has not moved to declare he would not support Trump if he wins the nomination fight.

“Governor Bush made explicitly clear on Tuesday his view that Donald Trump would be a chaos president who would be wrong for the country. Voters know where he stands. His focus is on defeating him in the primary,” the aide said.

Trump has said he does not want Bush’s endorsement.

Trump, the front-runner in national polls of Republican voters and leading in some of the early voting states, has taunted Bush all year.

Bush fought back with gusto at the debate, and the well-funded Super PAC that supports him, Right To Rise, released a television ad on Thursday that described the former Florida governor as "the one candidate tough enough to take on the bully." (here&feature=youtu.be)

It is to air in Iowa, which holds the first nominating contest of the 2016 election on Feb. 1, as well as the two next states to hold contests, New Hampshire and South Carolina.