Newt Gingrich has officially ended his campaign to become the Republican candidate to challenge US president Barack Obama in November's presidential election.
Mr Gingrich made the announcement to supporters in Arlington, Virginia.
The former House speaker won two primaries earlier this year but could not match Mitt Romney, who is now considered the presumptive nominee.
Mr Gingrich said November's election offered a choice between Mr Romney and Barack Obama, "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.
Reports have said Mr Gingrich ends his campaign some $US4m in debt.
He has pledged to take up the role of active citizen.
Mr Gingrich bowed out as the Obama re-election team issued a "greatest hits" video of Mr Gingrich's critiques on Mr Romney.
The end of Mr Gingrich's campaign leaves just Ron Paul as a notional challenger to Mr Romney.