Rick Santorum is looking to build on his surprising victories in three Republican contests and establish himself as a serious rival to Mitt Romney for his party's nomination to run against President Barack Obama.

Mr Santorum's victories in Minnesota, Colorado and Missouri jolted what has already been a tumultuous presidential race.

Before those contests, Mr Romney, a former Massachusetts governor, appeared on track for the nomination, with Newt Gingrich, a former speaker of the House of Representatives, as his main rival. Mr Santorum was seen as out of the running.

But Tuesday's results showed that Mr Romney is still struggling to win over the bulk of the conservatives who make up the bedrock of the Republican Party. And Mr Gingrich's weak showing, combined with Mr Santorum's success, made it unclear who the top rival is.

Mr Santorum, a former senator, said his victories led to a surge of donations. His few aides travelled to distant camps to start building campaign organisations from scratch.

Mr Santorum arranged a weekend of fundraising events in California and he planned to campaign in three other states next week. "We definitely are the campaign right now with the momentum, the enthusiasm on the ground," he said.

But he remains a long-shot for the nomination. Mr Romney has a lot more money and a much stronger campaign organisation. Mr Romney also leads the count of delegates who will choose the nominee, with 112 to 72 for Mr Santorum and 32 for Mr Gingrich.

Also, as long as Mr Gingrich and Mr Santorum are both viable, they could split the conservative vote, allowing Mr Romney to win nominating contests even if he falls short of a majority.

Mr Santorum's victory could benefit Mr Obama, who faces a tough re-election campaign because of the weak US economy.

Mr Romney is generally seen as his most formidable potential opponent. Even if Mr Romney ultimately prevails, a long, hard-fought nomination battle might weaken him ahead of the general election.