Via WaPo:

Rick Santorum will announce Tuesday that he is suspending his presidential campaign, all but bringing to a close the 2012 GOP presidential contest and formally handing the nomination to Mitt Romney, according to an official close to the campaign.

Santorum is set to announce his withdrawal at a campaign event happening shortly in Gettysburg, Pa. — the site of the historic and pivotal Civil War battle.

The former Pennsylvania senator had been Romney’s top opponent, but he suffered a trio of defeats last week in Wisconsin, Maryland and the District of Columbia, and over the weekend his daughter, Bella, was hospitalized for the second time this campaign due to complications from a rare genetic disorder.

Update:

Rick Santorum calls Mitt Romney to concede — Yahoo News

Rick Santorum called Mitt Romney Tuesday to say he is ending his presidential campaign, Yahoo News has learned.

Santorum is scheduled to make an announcement at a press conference in Gettysburg, Pa., at 2 p.m. ET.