"I will support whoever wins the Republican primary to beat Barack Obama," Santorum told supporters at a police gun range in northern Louisiana.
Poised to do well in Louisiana's primary Saturday, Santorum sought to explain his comment Thursday in Texas that Romney and Obama are so similar on the issues that Republicans might just as well vote to give the president a second term instead of casting their ballots for Romney.
Santorum argues that he is the only Republican candidate who can offer voters a stark contrast with Obama.
After testing his marksmanship with a .45-caliber semiautomatic Colt pistol, Santorum told reporters: "If you don't have a choice, then a lot of voters are going to vote for what they have. That's why we have to have a choice."
"I've said repeatedly and will continue to say, I'll vote for whoever the Republican nominee is and I will work for him," he added. "Barack Obama is a disaster, but we can't have someone who agrees with him on some of the biggest issues of the day."
While Santorum tamped down one fire, a supporter in the audience added an off-message wrinkle. As he fired the pistol, a woman in the crowd shouted: "Pretend it's Obama."
Santorum was wearing protective ear muffs. He said later that he didn't hear the remark but denounced it as "absurd."
"It's a very terrible and horrible remark and I'm glad I didn't hear it," he said.
The Secret Service, which provides security for Santorum, was trying to identify the woman.
"Typically, in an incident like this, we attempt to identify the individual, speak to them and then figure out what the next steps are," said spokesman Edwin Donovan. "People have a right to free speech but we have a right and an obligation to determine what their intent is."