A resurgent Rick Santorum has won the Republican presidential caucuses in Minnesota and Colorado, a stunning sweep that raised fresh questions about front-runner Mitt Romney's ability to appeal to the conservative core of the party's political base.

Mr Santorum, a former Pennsylvania senator also won a non-binding Missouri primary that was worth bragging rights but no delegates to the party's national convention.

Mr Romney, the former Massachusetts governor, prevailed in both Minnesota and Colorado in 2008, the first time he ran for the nomination, but the Republican Party has become more conservative in both states since then.

The victories for Mr Santorum, a fierce and vocal opponent of abortion and gay rights, exposed Mr Romney's long-time struggles to convince cultural conservatives that he is now in line with their beliefs despite his previous support of gay and abortion rights.

The victories were the first for Mr Santorum since he eked out a 34-vote win over Mr Romney in the lead-off Iowa caucuses a month ago.

Mr Santorum had faded far from the lead in the primaries and caucuses since Iowa, and Newt Gingrich, the former speaker of the House of Representatives, seemed to eclipse him as the leading conservative rival to Mr Romney when he won the South Carolina primary late last month.

The results were grim for Mr Gingrich, who made scant effort in any of the states that voted during the day. He ran far off the pace in the two caucus states, forced to watch from the sidelines while Mr Santorum boasted of being the candidate with conservative appeal.

Returns from 91% of Minnesota's precincts showed Mr Santorum with 45% support, Texas Representative Ron Paul with 27% and Mr Romney - who won the state in his first try for the nomination four years ago - with 17%. Mr Gingrich trailed with 11%.

It was closer in Colorado where returns from all the precincts showed Mr Santorum with 40% of the vote to 35% for Mr Romney. Mr Gingrich had 13% and Mr Paul claimed 12%. With all the precincts counted in Missouri, Mr Santorum had 55% of the vote, followed by Mr Romney with 25%. Mr Paul had 12%, while Mr Gingrich was not on the ballot.

But Mr Santorum remains a long shot for the nomination because Mr Romney has an overwhelming advantage in campaign funds, organisational strength and support from the party establishment needed to sustain what is now even more likely to be a drawn out state-by-state battle for the nomination.