Former Sen. Rick Santorum, R-Pa., received a big endorsement Thursday in his bid to win the Republican nomination for president.

Evangelical leader James Dobson announced his support for Santorum at a news conference in South Carolina.

Dobson is founder of the Colorado Springs, Colo.-based ministry, Focus on the Family. Dobson said he's supporting Santorum because he's putting social issues at the forefront of his campaign.

Santorum has urged conservatives to imagine what a head-to-head contest with President Obama will hold. He has tried to differentiate himself from Newt Gingrich and Mitt Romney as the clear conservative in the race.

He said both Gingrich and Romney had shared Obama's views on the Wall Street bailout and health care mandates in the past, which would mute any potential criticism of the president's policies during the campaign.

"How can you differentiate ourselves on the major issues of the day if we nominate Tweedledum and Tweedledee instead of someone who stood up and said, 'No'?" Santorum told conservatives in Charleston.

Santorum is currently polling at 4th place ahead of the South Carolina primary this Saturday.

Meanwhile, Virginia Gov. Bob McDonnell endorsed former Massachusetts Gov. Mitt Romney for president Friday.

"If you want to win the race in November, vote for Mitt Romney. He's the best candidate that we've got to beat President Obama," McDonnell said in a statement.