MCKINNEY, Texas (Reuters) - Republican presidential contender Rick Santorum, fresh off a stunning sweep of three states' nominating contests, scrambled in Texas on Wednesday to round up support and money he needs to take on rival Mitt Romney.
A day after winning contests in Minnesota, Missouri and Colorado, Santorum sought to build on his momentum, addressing Texas pastors, Tea Party movement activists and donors.
Santorum, a former U.S. senator and staunch social conservative, became the first Republican White House hopeful to win four of the state-by-state contests to pick a nominee to oppose Democratic President Barack Obama in the November 6 election.
His sweep on Tuesday raised new questions about the presumed front-runner Romney, who holds strong organizational and financial advantages but has yet to prove he can win over conservative Republicans who see him as too moderate.
Santorum took the field by surprise, days after being dismissed as an also-ran.
"Nobody ever thinks I can win anything," Santorum told about 600 people at a meeting with area pastors at the Bella Donna Chapel in McKinney, Texas. "The gift of being underestimated is a great gift."
Santorum is the latest in a series of rivals seen as the conservative alternative to the more moderate Romney, who needs to refocus to re-establish himself as the favourite for the nomination.
The surge came at a perfect time for Santorum, a Roman Catholic who speaks frequently about his seven children on the campaign trail and holds Christian conservative stances such as fierce opposition to abortion rights and gay marriage.
With signs of improvement in the U.S. economy, social issues have taken on more prominence in the 2012 campaign, helped by recent headlines.
'WAR ON RELIGION'
Republican contenders Romney, Santorum and Newt Gingrich repeatedly accuse Obama of waging war on religion because of positions including a rule requiring health insurance plans, including those offered by Catholic hospitals, to provide birth control.
Republican U.S. House of Representatives Speaker John Boehner joined the fray on Wednesday by saying the rule amounted to an attack on religious freedom and promising that Congress will act, if needed, to stop it.
A U.S. appeals court on Tuesday ruled that California's ban on gay marriage violated the U.S. Constitution. Santorum, Romney and Gingrich, a former House speaker, all denounced the decision.
The Susan B. Komen for the cure breast cancer charity's decision, later revoked, to stop funding for Planned Parenthood, also kept social issues in the public eye.
With Romney targeted as a "flip-flopper" for abandoning earlier moderate positions on healthcare and abortion, supporters credit Santorum for his unchanging positions.
"In a time when there is much cynicism about the authenticity of candidates, he has that box checked," Republican strategist Keith Appell said of Santorum.
Gingrich, the front-runner as recently as January 21, when he won the South Carolina primary, is struggling after campaign missteps and fierce attacks from Romney. He was not on the ballot in Missouri and was crushed in the other two states.
The next major Republican nominating contests are the Arizona and Michigan primaries on February 28, while Maine wraps up its caucuses this Saturday.
Critics portray Romney as a cold-hearted capitalist who cannot connect with voters because of his privileged upbringing as the son of a governor and corporate chief executive, and the personal fortune estimated at $270 million (170 million pounds) he amassed running a firm that bought - and sometimes broke up - troubled companies.
Some Christian conservatives are also wary of Romney because of his Mormon religion.
Santorum touts his background as the grandson of a coal miner, and says his economic policies would help generate jobs for working Americans.
Michael Gamble, a pastor from McKinney, said he was moved by Santorum's talk to the religious leaders about his family. "Today (Santorum) completely won me over," he said. "I think he represents the Kingdom of God."
MONEY AND ATTACKS
Backed by a wealthy "Super PAC" that pays for attack advertising against rivals, Romney has won in three of the eight states that have voted so far, including big wins in Florida and Nevada last week.
"Team Romney might need to tweak its strategy. So far they've been successful in going negative on their opponents and touting his business experience," Republican strategist Ford O'Connell said. "But obviously Republican primary voters are hungry for something more."
Santorum had also beaten Romney narrowly on January 3 in Iowa, a state where religious conservatives are a powerful force in Republican politics.
Romney's campaign says his business record makes him the best candidate to steer the sputtering economy. He scored points against Gingrich by targeting him as a Washington insider, and his campaign has been waging a similar attack on Santorum.
"Speaker Gingrich and Senator Santorum have over half a century's worth of time in Washington between them. They can't fix our country's spending problem because they helped create it," Romney spokeswoman Andrea Saul said.
Santorum said he was ready for an onslaught. His campaign was already bringing in more donations, an important consideration given how far he lags Romney in the money race.
In 2011, Santorum raised $2.2 million, according to year-end filings. Romney raised $56.8 million.
"We're doing very, very well raising money. I think last night we raised a quarter of a million dollars online," Santorum said on CNN.
In Minnesota's caucuses, Santorum won 45 percent of the vote. Texas Congressman Ron Paul was second with 27 percent and Romney was third at 17 percent, his lowest finish this year.
Santorum trounced Romney by 55 percent to 25 percent in Missouri. That vote was a non-binding primary, but has symbolic value as a measure of support in a big Midwestern state.
The race was closer in Colorado where Santorum won by 5 percentage points over Romney, 40 percent to 35 percent, but Romney had been expected to win easily.
(Additional reporting by Steve Holland in Denver, and Bill Trott, Susan Heavey, John Whitesides and Patricia Zengerle in Washington; Writing by Patricia Zengerle; Editing by Alistair Bell and Vicki Allen)