"We're still here. We're still fighting. We still believe, as this race really shows," Santorum told supporters in Green Bay, Wis.
Although the victory gives Santorum bragging rights and 10 more delegates, it does not change the overall dynamics of the race; the former Pennsylvania senator still dramatically lags behind Romney in the hunt for delegates to the GOP's summertime nominating convention.
Even so, Santorum's win underscores a pattern in the drawn-out race.
The under-funded underdog has tended to win in Bible Belt states that include Tennessee, Mississippi and Alabama. Romney - a deep-pocketed, highly organized former Massachusetts governor - has persistently struggled in such heavily conservative regions.
Said Santorum: "I'm not running as a conservative candidate for president. I am the conservative candidate for president."
Neither candidate was in the state as Louisiana Republicans weighed in. Nor was former House Speaker Newt Gingrich, who was trailing in Louisiana. With 100 percent of the precincts counted, unofficial returns showed Santorum with 49 percent to 27 percent for Romney. Gingrich was far back at 16 percent, followed by Ron Paul with 6 percent.
Romney tweeted his congratulations to Santorum: "Congratulations to Rick Santorum in LA. I look forward to the contests to come and to defeating @BarackObama in November."
Romney took a rare day off Saturday, with no public events. Santorum spent the day campaigning in Pennsylvania and next-up Wisconsin, which votes April 3 and represents one of his last chances to beat Romney in a Midwestern state.
Santorum told voters in Milwaukee that he expected their state to be "the turning point in this race."
In an unmistakable jab at Romney, Santorum added: "Don't make the mistake that Republicans made in 1976. Don't nominate the moderate. When you do, we lose." It was a reference to Ronald Reagan losing the 1976 Republican nomination to incumbent President Gerald Ford, and Democrat Jimmy Carter winning the White House.
Exit polls conducted for The Associated Press and the television networks showed that Santorum's win in Louisiana was one of his strongest performances to date among conservatives, working class voters and those calling the economy their top issue. And he continued his dominance among white evangelical voters and those looking for a candidate who shares their religious beliefs. Santorum topped Romney among evangelical voters by more than 2 to 1.
As in previous Southern states, Romney's best showing came among those voters with annual incomes above $100,000 and those who prioritized a candidate's ability to defeat President Barack Obama in November.
The bad economy was the top issue for Louisiana voters. Most were gloomy about prospects for a recovery, saying they felt the economy was getting worse instead of better. While some national surveys suggest Americans are feeling optimistic about economic improvement, just one in eight Republican primary voters said they thought a recovery was under way.
Romney is far ahead in the delegate count and on pace to reach the necessary 1,144 delegates before the party's convention in August.
With the Louisiana results, Romney leads the overall race for delegates with 568, followed by Santorum with 273, Newt Gingrich with 135 and Ron Paul with 50.
Santorum badly needed a rebound after a decisive Illinois loss to Romney earlier in the week that moved party stalwarts to rally around the front-runner. Many urged Santorum and Gingrich to drop out of the race.
Both refused, and campaigned aggressively in Louisiana in hopes that a victory there would justify them staying in - despite Republican worries that the long nomination fight could hurt the party's chances against Obama. The Democratic incumbent faces no serious primary challenge and his re-election campaign already is well under way.
Romney barely campaigned in Louisiana, though his allies spent on TV ads there. Instead, Romney was looking past the results and toward the general election.
"I want the vote of the people of Louisiana so we can consolidate our lead," Romney said Friday during a stop in Shreveport. He told supporters his campaign wants to focus on "raising the money and building the team to defeat someone that needs to be out of office in 2012, and that's Barack Obama."
Earlier Saturday, Santorum said he wanted to debate Romney without their trailing competitors on stage.
"This race has clearly gotten down to two candidates that can win the nomination," Santorum told reporters in Milwaukee. "I'd love to have a one-on-one debate."
In the run-up to Louisiana's voting, Santorum found himself on the defensive after suggesting he'd prefer a second term for Obama over a Romney presidency. Santorum was all but forced to walk back those comments, saying less than 24 hours before Louisiana polls opened that "over my dead body would I vote for Barack Obama."
Romney also faced trouble last week when a top aide compared the switch from a primary to a general election campaign to an Etch A Sketch toy, suggesting earlier campaign positions could be easily wiped away.
But most Louisiana voters said they weren't concerned with the comment, with only about one in five in exit polls calling this week's Etch A Sketch controversy an important factor in their vote.
Louisiana has complicated delegate rules: Even though there were 20 delegates at stake Saturday, they are awarded proportionally to the candidates who receive more than 25 percent of the vote.
Most states divide all the available delegates among the candidates who meet the minimum threshold. Louisiana's system is strictly proportional, with any leftover delegates designated as uncommitted, meaning they will be fought for at the state convention.
The next key fight comes April 3 in Wisconsin. Romney's campaign is airing TV ads in the state, and his super PAC allies have plowed more than $2 million into TV advertising there.
Also voting April 3 are Maryland and the District of Columbia. There are 95 delegates combined at stake in the three contests.
Elliott reported from Milwaukee. Associated Press writer Marc Levy in Camp Hill, Pa., contributed to this report.