(CNN) - Newt Gingrich is not backing down on rival Rick Santorum, who sparked controversy when he said the country might be better off with President Barack Obama in a second term than with Mitt Romney as president.
In a not-so-veiled swipe at Santorum Friday, Gingrich's chief of staff, Patrick Millsaps, issued a letter to the Republican National Committee, urging its chairman to ask all GOP presidential candidates to pledge their support for the eventual Republican nominee.
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"It is imperative that Republicans unite once the nomination process is complete in order to defeat President Obama. We cannot afford four more years of his leadership," Millsaps said.
Santorum's original comment came Thursday in a San Antonio speech, in which the candidate said Obama and Romney had so few differences that "we may as well stay with what we have instead of taking a risk in what may be the Etch A Sketch candidate for the future."
Romney's campaign quickly hit back against the comment, sending out a statement condemning the remark.
"I was disappointed to hear that Rick Santorum would rather have Barack Obama as president than a Republican," Romney said in the statement. "This election is more important than any one person."
On Friday, the former Pennsylvania senator asserted he would "never" vote for Obama over any Republican and to suggest otherwise would be "preposterous."
"I was simply making the point that there is a huge enthusiasm gap around Mitt Romney and it's easy to see why – Romney has sided with Obama on healthcare mandates, cap-and-trade, and the Wall Street bailouts," Santorum said in a statement.
Despite Santorum's statement, Gingrich's campaign continued to keep the remark in the spotlight with its letter to the RNC on Friday.
"Republicans must not lose sight of our ultimate goal in 2012: defeating President Obama in November," the letter stated.
"While we may disagree on which candidate will be the strongest opponent to the President in the general election, we can agree that any of the current Republican candidates would be a better president than Barack Obama."
- CNN Chief Political Correspondent Candy Crowley contributed to this report.