Media playback is unsupported on your device Media caption Newt Gingrich outlines his views on why Mitt Romney is preferable to Barack Obama

Newt Gingrich has formally ended his presidential campaign, but fell short of an endorsement for presumptive Republican nominee Mitt Romney.

He said November's election offered a choice between Mr Romney and Barack Obama, "the most radical, leftist president in American history".

Mr 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 Mr Gingrich's critiques on Mr 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".

Written off

Mr Gingrich said Mr Romney was a preferable conservative alternative to Mr Obama.

Analysis 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. He spent most of the time in a detailed review of the highlights of his own career and future plans as a conservative sage. It had been expected that he would endorse Mitt Romney. But blink and you would have missed him even talking about the presumptive Republican nominee. I don't think the grudging acceptance that he was marginally better than the man in the White House really qualifies as an endorsement. It was so lacking in enthusiasm that it was several notches on the thermometer down from lukewarm. In effect, it was telling conservatives who may not be happy with Mr Romney that they are stuck with him and have nowhere else to go. This may be the sober assessment of many Republicans. The Romney camp may be annoyed by their former rival's brusqueness, but they should not be too bothered. After all, Mr Gingrich is leaving the race because he has not got enough support. There are hardly legions hanging on his every word. But the Democrats will be delighted. They are already making the most of Mr Gingrich's "wild ride". They have put out a video featuring the former speaker calling the former governor a liar, and a man who will not keep his word.

"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."

Mr Gingrich added that Mr Romney knew "about 60,000 times more" on the subject of job creation than Mr 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," Mr 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 Mr Romney.

The Georgia politician's exit provided the Obama campaign with the opportunity to highlight Mr Gingrich's regular primary-season attacks on the presumptive nominee.

The edited compilation suggests Mr Gingrich does not support the Romney campaign or its key positions.

In his own video message , released earlier on Tuesday, Mr Gingrich did not mention Mr Romney.

Mr Romney and supporting groups used large numbers of negative advertisements against Mr 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.

Mr 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.

But a series of strong debate performances brought him back to prominence, and he won the South Carolina primary in January.

He also won the primary in his home state of Georgia but failed to build momentum during the primary season and struggled to match Mr Romney's financial muscle.