Santorum, who most of the year lagged in polls and campaign cash before surging in Iowa, took aim at Romney in a fund-raising appeal.
"Now is the time to act or get stuck with a bland, boring career politician who will lose to Barack Obama," he wrote.
On Thursday, the former Pennsylvania senator targeted a Massachusetts law that Romney signed that requires everyone to buy health insurance, just as the 2010 federal law backed by Obama does.
'Make a run'
Newt Gingrich, the former House speaker who also seeks the GOP nomination, also once supported the mandating of coverage as part of a overhaul. He has reversed that position.
"I've never been for government-run health care - never - unlike the other two folks who are running here that supported individual mandates, that supported top-down government health care," Santorum said.
Still, Santorum sought to manage expectations Thursday.
"I mean, the chances in five days to make up a 35- or 40-point lead is going to be pretty limited," he told reporters earlier in the day in Manchester. "But we expect to make a run and to move up in those polls and to show that we're the candidate with the momentum, and we'll carry that into South Carolina," which holds its presidential primary on Jan. 21.
Voter concerns
Romney has the backing of 41 percent of likely Republican primary voters in New Hampshire, according to a two-day Suffolk University/7 News tracking poll released Thursday. U.S. Rep. Ron Paul of Texas was at 18 percent, compared with 8 percent for Santorum and 7 percent for both Gingrich and former Utah Gov. Jon Huntsman Jr.
Santorum had to address some voters' concerns. One man in Tilton, worried about gun rights, confronted Santorum about his support for former Pennsylvania Sen. Arlen Specter, who backed gun control and left the GOP to become a Democrat.
Santorum said he is committed to gun rights and acknowledged his support for Specter.
"Am I perfect? No," he said. "I've made mistakes, and I've been upfront about that."
In Manchester, Santorum campaigned as if he were already the nominee with rhetoric focused on Obama.
"We have a president who doesn't understand us," Santorum said at a Rotary Club meeting.
He said Obama has run roughshod over the Constitution's separation of powers doctrine by installing the new head of the federal consumer protection agency without Senate approval. He also spoke against the recess appointments of three new members to the National Labor Relations Board.
"You are not above the law, Mr. President," Santorum said. "We don't want the NLRB to be filled with a bunch of cronies who work for the labor unions."
This article contains information from Bloomberg News and McClatchy Newspapers.
