Republican Mitt Romney, who dropped out of the presidential race a week ago, will endorse one-time rival Sen. John McCain, Republican officials said on Thursday.

Romney and McCain had tangled bitterly during the campaign as they fought for their party's nomination and Romney, a former governor of Massachusetts, attracted many of the party's conservative base to his side.

After a string of losses Romney dropped out of the race last Thursday saying it would be best to unify the party to prepare for a tough general election battle against either of the two Democrats, Barack Obama or Hillary Clinton.

(Article continues below)

"Romney has decided to endorse McCain and it could happen as early as this afternoon," said a Republican Party source familiar with the situation.

Kevin Madden, a spokesman for Romney, said the endorsement would take place at 4 p.m. at Romney's campaign headquarters in Boston.