Rick Santorum formally joins GOP presidential field

Former Sen. Rick Santorum on Monday became the latest Republican to formally declare for the 2012 presidential nomination.

Like rival Mitt Romney, who announced his bid Thursday, Santorum’s entry had been expected for some time.

“We’re going to be in this race, and we’re in it to win,” Santorum said on ABC’s “Good Morning America.” He said his campaign has a “lot of momentum” in the early primary states.

Santorum planned to give a speech later Monday morning in western Pennsylvania, near where his immigrant Italian grandfather worked in coal mines.

On ABC, host George Stephanopolous asked a question which has been on the mind of many political observers: How can a man who lost his last Senate race (to Bob Casey) by 18 points make a credible case for the White House?

Santorum responded by saying he has stuck to his core conservative principles. He blamed his loss in 2006 on a poor electoral climate for Republicans.

Best known for his strong stance on social issues, Santorum also plans to be a loud voice in the room on entitlement reform. Not only did he renew his call for reforming Social Security on Monday, he said that the Paul Ryan Medicare revamp plan, which Republicans have been handling delicately ever since it emerged as a volatile and polarizing political issue this spring, doesn’t go far enough because it wouldn’t affect current seniors.

Santorum, 53, has shown little strength in polls, routinely scoring in the low single digits. He served two terms in the House and two terms in the Senate before his 2006 loss to Casey.

james.oliphant@latimes.com