GETTYSBURG, Pennsylvania (Reuters) - Rick Santorum ended his improbable run for the White House on Tuesday after leading a Republican tilt to the right that could dog the more moderate front-runner, Mitt Romney, in November's election.
Trailing in polls and fundraising, the conservative former Pennsylvania senator suspended his campaign and cleared the way for Romney to clinch the nomination to face President Barack Obama in the November 6 general election.
A staunch social conservative with a penchant for sleeveless sweaters, Santorum was the underdog who clawed his way to near the top of the Republican race and won the first 2012 nominating contest in Iowa by a thread.
His rise forced the issues of birth control and the role of Christianity in public life to the forefront of the campaign, frustrating Romney, a former private equity executive who sought to keep the focus on the economy.
During his exit speech on Tuesday, Santorum again reached out to the working class and the Republican Party's right wing, which he had courted throughout the campaign with his focus on manufacturing, religion and conservative family values.
"Over and over again we were told, 'Forget it, you can't win.' We were winning, but in a different way. We were touching hearts; we were raising issues that frankly a lot of people didn't want to have raised," Santorum said at a news conference in a hotel near the Civil War battlefield site of Gettysburg.
Santorum's exit leaves the stage free for a two-man fight between Romney and Democrat Obama for the presidency.
That contest intensified on Tuesday when Obama's campaign accused the multimillionaire Romney of not paying his "fair share" of taxes, and tried to paint the Republican as an elitist.
Romney campaigned in Wilmington, Delaware, where he met with women who owned small businesses and called Obama's handling of the economy a failure.
On Santorum, Romney said the news about his rival was unexpected. He added that Santorum has been an important voice and will continue to play a major role in the Republican Party.
"This has been a good day for me," Romney said.
Santorum proved to be a more formidable opponent to Romney than many expected, especially in light of a historic 18-percentage point defeat during his Senate re-election bid in Pennsylvania in 2006.
His vocal opposition to gay marriage and abortion offered Republican voters a stark contrast with former Massachusetts governor Romney's more moderate record.
Romney moved to the right on social issues to try to outflank Santorum. Now Romney, and Republican congressional candidates, could have some difficulty in November, when the overall electorate will be more moderate.
"When voters are interested in the economy, Rick Santorum was talking about socially conservative issues ... and that would take us off message. That would take the whole party off message," said Republican strategist Ron Bonjean.
Santorum lagged Romney in opinion polls and in the fight for the 1,144 party delegates needed to win the Republican nomination. He was facing the possibility of an embarrassing defeat in his home state of Pennsylvania on April 24. Romney has 659 delegates to 275 for Santorum, according to a CNN estimate.
A devout Catholic, Santorum failed to stretch his appeal far enough beyond conservatives and some blue-collar Republicans to overtake Romney.
Santorum spoke to his main rival on Tuesday but did not endorse the front-runner or either of the two other Republican candidates, Texas congressman Ron Paul and former U.S. House Speaker Newt Gingrich, both of whom are way behind Romney in polls.
"Mr. Santorum brought excitement to the race, and helped the GOP mobilize voters earlier in the season," Hunter College Political Science Professor Jamie Chandler said.
"Now much of the electorate will tune out until the fall. His delegates will now be open at the convention, but will likely support Mitt Romney."
Disappointed Santorum supporter Felicia Collie, 29, of Gettysburg, was not ready to throw her vote to Romney.
"I don't really want to because Romney is the same as Obama," she said. "Santorum is the only one (of the Republican candidates) who is a clear contrast."
Santorum has been the only serious Republican challenger to Romney in the last six weeks, and he won victories in nominating contests in Alabama, Mississippi, Missouri, Kansas, Louisiana and Colorado.
But he failed to take big Rust-Belt states such as Michigan and Ohio, despite portraying himself as a blue-collar guy and playing up his roots as the grandson of an Italian immigrant coal miner.
Santorum also could not compete in campaign funding.
Romney's allies in the Restore Our Future "Super PAC" have spent nearly $41 million on advertising that consisted mostly of attacks on Gingrich and Santorum, portraying the latter as a supporter of big spending during his 12 years in the Senate.
"It's no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads," said Obama's campaign manager Jim Messina.
"But neither he nor his special-interest allies will be able to buy the presidency with their negative attacks. The more the American people see of Mitt Romney, the less they like him and the less they trust him," said Messina, who heads a campaign that has a stronger fundraising operation than Romney's.
Santorum leaves a legacy of harsh statements on social issues that alienated many voters, including Republicans.
He upset fellow Catholics in February when he said that late President John F. Kennedy's 1960 speech praising the separation of church and state made him want to "throw up." Santorum later said he regretted the comment.
When Santorum criticized gay rights and birth control, the media highlighted a 2008 speech in which he declared that "Satan" was attacking great U.S. institutions by means of "pride, vanity, and sensuality."
The father of seven children, Santorum's decision to quit was partly influenced by a serious illness suffered by his 3-year-old daughter, Bella. She was hospitalized over the long holiday weekend with Trisomy 18, a rare genetic condition that hinders a child's development.
(Additional reporting by Alina Selyukh, Andy Sullivan, Jeff Mason and Sam Youngman; Writing by Alistair Bell; Editing by Mary Milliken, David Brunnstrom and Lisa Shumaker)