Newt Gingrich has formally ended his presidential campaign and is expected to endorse the presumptive Republican nominee, Mitt Romney.
After losing contests in five states last week, Gingrich said on Wednesday it was clear that Romney would be the nominee and he signaled that his own topsy-turvy campaign had reached its end.
The former US House of Representatives speaker, the face of the Republican party in the mid-1990s, badly trailed Romney in polls and his campaign fell into debt of $4.3m.
Gingrich dropped his White House bid at a news conference in Arlington, Virginia, although he had not been campaigning properly for weeks after plunging in polls and cutting staff.
His withdrawal leaves only Ron Paul, a Texas congressman who represents the libertarian wing of the party, in the race against Romney.
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While Romney, 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, author of the "Contract With America" that helped Republicans win back control of Congress in 1994, won primaries in South Carolina and his home state of Georgia this year but failed to make a mark in larger states like Florida and Ohio.
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.
Gingrich's run turned some of his more bizarre ideas, like establishing a US moon colony and having school children work as janitors, into media fodder.