Former U.S. senator Rick Santorum rejuvenated his presidential hopes on Tuesday with overwhelming victories over front-runner Mitt Romney in Republican nominating contests in Missouri and Minnesota.

Romney was leading the conservative Santorum in Tuesday's other vote in Colorado, where results were still being tallied in that state's unexpectedly close presidential caucuses and no winner had yet been determined.

Backed by a wealthy Super PAC that pays for attack ads against rivals, Romney had excelled in major contests thus far in the race but he did little campaigning in Minnesota and Missouri.

Until Tuesday, Santorum had won only one of the first five Republican contests in the state-by-state battle for the Republican nomination to face President Barack Obama in the November 6 election.

But on the first day of multiple nominating contests in the 2012 primary season, Santorum was the winner in Missouri, where he had 55 percent of the vote to Romney's 25 percent, with nearly all the ballots counted. The Missouri vote was a non-binding primary, but has symbolic value as a measure of support in a big Midwestern state.

In Minnesota's caucuses, Santorum had 45 percent of the vote, with 82 percent of the returns counted. In another blow to former Massachusetts governor Romney, U.S. congressman Ron Paul was in second place in Minnesota with 27 percent and Romney was a distant third with 17 percent.

It was the first time so far in the 2012 Republican race that Romney did not come in first or second in a state contest, and Tuesday's results may force him to recalibrate and focus more on attacking Santorum.

The former Pennsylvania senator is a devout Catholic who is in a battle with former U.S. House of Representatives speaker Newt Gingrich to become the conservative alternative to the more moderate Romney.

His victories give heart to social conservatives fighting battles of abortion, gay marriage and contraception in recent days.

Wow. Conservatism is alive and well in Missouri and Minnesota, Santorum told supporters in St. Charles, Missouri after results came in.

I don't stand here to claim to be the conservative alternative to Mitt Romney, I stand here to be the conservative alternative to Barack Obama, he added to cheers from the crowd.

Santorum has rebuilt a political career left in tatters in 2006 when voters in Pennsylvania threw him out of the Senate by an 18 percentage point margin. But he still faces an uphill battle for the presidential nomination, considering Romney's vast organizational and financial advantages.

Gingrich was not on the ballot in Missouri, allowing Santorum the chance to consolidate conservative voters in the state and compete directly with Romney and Paul, known for his libertarian views.

'BEAUTY CONTEST'

The Missouri primary is considered a beauty contest because the candidates do not win delegates who will take part in the August Republican convention where the party's presidential nominee will officially be chosen. Missouri Republicans will select convention delegates in caucuses on March 17 in the state's two-step process.

In caucuses, voters gather in public places, listen to speeches from representatives of the various candidates and cast ballots for their choice.

Santorum has made his strong positions on social issues a centrepiece of his bid and is hoping his staunch opposition to gay marriage and abortion will help win over conservatives wary of Romney because of moderate positions he took while running for office in liberal Massachusetts.

Romney, who also previously headed the private equity firm Bain Capital, is the best funded and best organized of the Republican candidates vying for a shot at Obama in the general election. And he was coming off big victories in Florida and Nevada in the past week.

But many Republican voters, particularly the most conservative ones, have failed to embrace him. Some Christian conservatives are wary of Romney because of his Mormon religion.

Santorum's campaign is far behind Romney in fund-raising and in support by the Republican establishment, but he has managed to win followers - particularly in conservative Midwest states - with his message on social issues.

Tuesday's strong showing by Santorum came as divisive social issues came to the forefront again in U.S. politics. Earlier in the day, a U.S. appeals court ruled that California's ban on gay marriage violated the U.S. Constitution in a case expected to lead to a showdown in the Supreme Court.

In addition, U.S. Roman Catholic bishops have slammed the Obama administration for a new regulation that would require health insurance to include birth control. The bishops contend the policy infringes on religious liberty because the church does not condone birth control of any kind.

Santorum took some jabs at Obama, focusing in particular on the new contraception rule. He accused the president of trying to impose his secular values on the people of this country.

The next major Republican nominating contests are the Arizona and Michigan primaries on February 28, while Maine wraps up its caucuses this Saturday.

A Reuters/Ipsos poll on Tuesday that has Romney ahead in the race nationally with 29 percent showed Santorum's support has gained by 5 percentage points in the last month, to 18 percent.

That put him in a virtual tie with Gingrich, at 19 percent, and Paul, who was at 21 percent.

(Additional reporting by Lily Kuo in Washington and Steve Holland in Colorado; Editing by Will Dunham)