US congressman Ron Paul has said he is suspending active campaigning in the race for the White House.
Mr Paul said he will continue to work on amassing delegates to the party's national convention, even though his chances of winning the nomination are virtually nil.
He said his team will seek to win delegates in order to "carry a strong message" to the Republican National Convention in Tampa, Florida in late August.
A Republican candidate needs 1,144 delegates in order to be declared the nominee.
Mr Romney's two main challengers, Rick Santorum and Newt Gingrich, dropped out in April and May, respectively, leaving Mr Paul as Romney's lone Republican opponent for the past two weeks.
After coming third in Iowa and second in New Hampshire - the first votes to choose the Republican who will challenge President Barack Obama in November - the 76-year-old shot unexpectedly into the mainstream.
Few believed that Mr Paul could translate his support into enough delegates to win the nomination, but party elders have been careful not to trample his ideas in case he bolts altogether and launches a third-party presidential bid.
