Washington: Republican Mitt Romney bowed out of the 2016 US presidential race on Friday after considering a third run, telling supporters it was time for the next generation of party leaders to seek the White House.

Romney's decision helps clarify an emerging Republican field split between potential establishment candidates like former Florida Governor Jeb Bush and conservative voices represented by Texas Senator Ted Cruz and former Arkansas Governor Mike Huckabee.

The move will likely boost Bush, the brother of former President George W Bush and son of former President George HW Bush, even though Romney has privately made known to aides he is not convinced that Bush can defeat Hillary Clinton if she becomes the Democratic presidential nominee.

But Romney's decision also helps other potential candidates such as Florida Senator Marco Rubio, who was on Romney's vice presidential short list in 2012; Wisconsin Governor Scott Walker and New Jersey Governor Chris Christie, all of whom will have a better chance for money and media attention with Romney on the sidelines.

The former Massachusetts governor felt he would win the Republican presidential primary, a former adviser said. But Romney did not think he would present a sufficient ‘new versus old’ contrast for the battle against Clinton, the former first lady, New York senator and secretary of state, who is the early favourite for the Democratic nomination.

Romney, the 2012 Republican nominee, announced he would not run in a statement he read to supporters in a conference call from New York.

"After putting considerable thought into making another run for president, I’ve decided it is best to give other leaders in the party the opportunity to become our next nominee," Romney said in his statement.

He made clear he felt he would be able to raise enough money for a campaign, rejecting a narrative that has grown in recent days as some major 2012 fund-raisers expressed concern about another Romney run.

Those fund-raisers said that since Romney opened the door to a possible candidacy three weeks ago, he had not made clear how his message and inner circle would improve in 2016.