Newt Gingrich, has pulled out of the race
Newt Gingrich has formally ended his presidential campaign, but fell short of an endorsement for presumptive Republican nominee Mitt Romney, reports the BBC.
He said November's election offered a choice between Romney and Barack Obama, "the most radical, leftist president in American history".
Gingrich ends his campaign some $4m (£2.5m) in debt, according to reports.
He bowed out as the Obama re-election team issued a "greatest hits" video of Gingrich's critiques on Romney.
The former House Speaker told supporters in a speech at a hotel in Arlington, Virginia, that "today I am suspending the campaign. But suspending the campaign does not mean suspending citizenship".
Newt Gingrich's speech was a connoisseur's delight. There is no reason in the former Speaker's book why an admission of defeat should also be a moment of humility.
"I am asked sometimes, is Mitt Romney conservative enough?" he said. "My answer is simple: compared to Barack Obama?
"This is not a choice between Mitt Romney and Ronald Reagan. This is a choice between Mitt Romney and the most radical, leftist president in American history."
Gingrich added that Romney knew "about 60,000 times more" on the subject of job creation than Obama.
The former House Speaker outlined policy initiatives he hopes to support, including research in regenerative medicine, protecting religious liberty and the oft-mentioned moon colony.
"My wife has pointed out to me that the moon colony was not the most clever comment in this campaign," Gingrich said.
"But the underlying key point is real: if we're going to be the leading country in the world, we're going to have to be the leading country in space."
The end of his campaign leaves just Ron Paul as a notional challenger to Romney.
The Georgia politician's exit provided the Obama campaign with the opportunity to highlight Gingrich's regular primary-season attacks on the presumptive nominee.
The edited compilation suggests Gingrich does not support the Romney campaign or its key positions.
In his own video message, released earlier on Tuesday, Gingrich did not mention Romney.
Romney and supporting groups used large numbers of negative advertisements against Gingrich and former Republican rival Rick Santorum in the primaries.
While the former Massachusetts governor is not the official Republican nominee, he is on track to gain the 1,144 delegates needed to secure the nomination by the end of May.
Gingrich's White House run was full of incident. He was largely written off in the summer of 2011 when a number of key campaign staff walked out on him.