Mitt Romney, the Republican beaten by President Obama in the US 2012 election, has decided he will not run for president again.
Mr Romney, 67, said he had decided it was “best to give other leaders in the party the opportunity to become our next nominee”.
His statement comes weeks after a surprise announcement saying he was considering another bid.
The decision not to run frees up donors to support other Republican candidates.
Former Florida Governor Jeb Bush, New Jersey Governor Chris Christie and Senator Rand Paul are among those who are considering a White House bid.
On the Democratic side, the presumed front-runner is former Secretary of State Hillary Clinton, although she has not made any announcement about campaigning.
Mrs Clinton previously ran for president in 2008 but lost the Democratic nomination to Mr Obama.
2016 runners and riders
No-one has formally declared but these are some of the names to watch:
- early Republican frontrunner is Jeb Bush
- but New Jersey Governor Chris Christie could battle Bush for the party’s centre ground
- darling of the Tea Party is Texas Senator Ted Cruz
- firebrand liberal Elizabeth Warren is championed by many in the Democratic Party
- libertarian Rand Paul has his supporters – and enemies – among Republicans
- Hillary Clinton will have learnt much from her failed campaign of 2008