From the AP:

Republican campaign dropout Mitt Romney agreed Thursday to endorse Sen. John McCain for the party’s presidential nomination and ask his national convention delegates to swing behind the party front-runner, according to officials familiar with the decision.
Romney collected 280 delegates during his run through the early primaries and caucuses, more than enough to put McCain over the 1,191 needed to clinch the nomination.

The officials who disclosed Romney’s plans did so on condition of anonymity because the formal announcement is expected later in the day.

Romney’s decision marked a harmonious end to an occasionally contentious struggle between the two men over the party’s presidential nod. They criticized one another in television ads in state after state, a clash that effectively ended on Feb. 5, when McCain won a string of big-state primaries from coast to coast.

Officials said the former Massachusetts governor made his decision to back McCain earlier in the day, citing a desire to help the Arizona senator wrap up the nomination before too much more time passed.