Despite this big loss, 65-year-old Romney is still considered to be the frontrunner as he has the maximum number of delegates (560), with Santorum a distant second (254) followed by Newt Gingrich who has 141 delegates with him.
The candidates need 1,114 delegates to be able to earn the Republican Party's nomination to challenge incumbent Barack Obama in the November presidential elections.
Jubilant by the Louisiana victory, 53-year-old Santorum, a former Pennsylvania senator and social conservative, said Saturday's results have shaken up the Republican presidential primary, adding that it is a two-way race now.
"We're here to fight. This race is long and far from over," Santorum said.
"The entire Santorum campaign, and the conservative movement, is excited by tonight's results, that stand in the face of the media's and the establishment's declaration that we must fall in line with a moderate from Massachusetts," said Hogan Gidley, the national communications director of the Santorum campaign.
"There is no other way to interpret tonight's results but as vindication for Rick Santorum's consistent, conservative message of strengthening our nation based on our founding values," Gidley said.
"Louisiana voters overwhelmingly rejected Mitt Romney's push to press the reset button, because they know that we need a clear contrast to President Obama's disastrous policies," the spokesperson added.
In a statement, Gingrich said Santorum deserves credit for coming in first in Louisiana, but was quick to add that the race is still open.
"This is clearly still an open race. So, on behalf of the more than 176,000 Americans who have donated to Newt 2012, I will carry our solution-oriented campaign to Tampa," Gingrich said after the results of Louisiana were declared.
"Romney's defeat represented the latest setback in the South for the front-runner, who has lost primaries in six Southern states this month and also lost a key primary earlier this year in South Carolina," the Washington Post said.
"Santorum, meanwhile, can again claim momentum based on a strong showing in a heavily conservative state. He has won seven contests for the Republican presidential nomination this month, including five in the South," the daily said.
