Conservative Republican Rick Santorum has endorsed Republican presumptive presidential nominee Mitt Romney in an email to supporters, US media reported late Monday.
During his own push for the Republican nomination Santorum, a staunchly Roman Catholic former senator who withdrew from the presidential race last month, said Romney was not conservative enough.
Now however, he has told his supporters to back Romney to beat Democratic President Barack Obama in November, The Los Angeles Times and other media reported.
Romney and Santorum met on Friday to do some fence mending after a bitter campaign, but Romney said then he was not expecting his former rival's formal backing yet.
At campaign events, arch-conservative Santorum repeatedly spoke about social themes such abortion and gay rights, two bedrock conservative issues on which Romney has changed his position over the years.
When Santorum dropped out of the race April 10 after a surprisingly strong grass roots campaign that began last year in a pick-up truck, he did not mention Romney's name.
But the Republicans must close ranks if they want to keep Obama from winning reelection in November.
Newt Gingrich, a diehard challenger for the Republican nomination, officially quit the campaign last week and endorsed Romney.
Libertarian-leaning congressman Ron Paul remains in the race, but he has no chance of defeating Romney.