Republican presidential candidate, Mitt Romney
Republican frontrunner, Mitt Romney faces a tough battle to become the party's White House contender after rival Rick Santorum cemented his position as the conservative alternative with double wins in the Deep South.
While Romney has a commanding lead halfway through the state-by-state nominating process which began in January, his failure to seal the deal with the party's core conservatives has exposed an underlying weakness, reports AFP.
It also seriously undermines claims that Romney is the Republicans' best shot at beating President Barack Obama in the November elections.
The "fratricidal" Republican primary process is also providing Obama's Democrats with plenty of fodder for winning over independents in key battleground states, said political expert Rogers Smith.
"If the national economy is doing poorly the Republican candidate may well win, but there's little question that if that's not the case the Republican selection process will probably end up having benefited Obama," Smith, from the University of Pennsylvania, told AFP.
A Pew poll released Wednesday showed Obama with a 12-point advantage over Romney in a national match-up and a whopping 18-point lead over Santorum.
The battle to win over the Republican conservative base has re-ignited America's divisive culture wars over issues such as abortion, something which has helped Santorum but could hurt Republicans come November 6.
"Party donors and insiders dread a Santorum candidacy as likely to lead to a 300+ electoral vote Obama blow-out," conservative pundit, David Frum wrote on the Daily Beast Wednesday.
"The Santorum candidacy pushes Republicans toward an election in which the issues are religious, cultural, and sexual, not economic."
Santorum's dramatic come-from-behind victories in southern Alabama and Mississippi on Tuesday will keep social issues high on the agenda and continue to pull Romney to the right, analysts said.
It has also renewed pressure on former House speaker and southern native Newt Gingrich to bow out of the race, as the two states were seen as must-wins for his flagging campaign.
Gingrich has won only two out of about 29 contests held in states and territories, but has accumulated an estimated 139 delegates thanks to proportional distribution.
It takes 1,144 delegates to clinch the nomination, but Republicans have historically picked their standard-bearer long before that number is reached.
Santorum has 239 delegates and has come out on top in nine contests while Romney's 19 wins helped him collect 495 delegates, according to CNN's estimates of the complex delegate distribution process.