By Steve Holland

WASHINGTON (Reuters) - Rick Santorum tried to raise doubts about the electability of Mitt Romney on Friday at a conservative gathering where he openly attacked his chief rival for the Republican presidential nomination.

Buoyed by three victories this week, Santorum presented himself as the true conservative in the 2012 race and said Romney's more moderate record does not draw enough of a contrast with Democratic President Barack Obama.

Santorum criticized the view held by many Republicans that while Romney is not the most conservative hopeful, he would be a strong candidate in the November 6 election against Obama because he would appeal to swing voters.

"Why would an undecided voter vote for a candidate that the party's not excited about?" Santorum told the Conservative Political Action Committee forum.

Santorum's wins in Colorado, Minnesota and Missouri on Tuesday have given the former Pennsylvania senator a burst of momentum and a reason to promote himself as Romney's main conservative alternative in the weeks leading up to March 6, the Super Tuesday when 10 states vote.

Conservatives, Santorum said, "need conservatives now to rally for a conservative, to go into November to excite the conservative base, to pull with that excitement moderate voters and to defeat Barack Obama in the fall."

Romney was to address the group later in the day along with former House of Representatives Speaker Newt Gingrich.

Surrounded by his family on stage, Santorum took Romney on without naming him, questioning his conservative credentials, particularly on healthcare and in support of bank bailouts during the 2008-09 financial crisis.

Romney has taken steady fire during the campaign because as governor of heavily Democratic Massachusetts he helped develop a healthcare plan that the White House has cited as a model for the Obama overhaul, which conservatives want to repeal.

Romney has vowed to repeal the Obama plan if elected.

"Who has supported in fact the stepchild of 'Obamacare,' the person in Massachusetts who built the largest government-run healthcare system in the United States?" Santorum said.

If Romney is the Republicans' nominee, he would "simply give away that issue" in the campaign against Obama, said Santorum.