Now that Rick Santorum has suspended his campaign, Mitt Romney is reaching out to conservative activists. But his reception so far has ranged from tepid to downright hostile.

“To date, Mitt Romney has spent about $100 million to drive the conservative candidates from the field, in some case through personal attacks,” conservative organizer Richard Viguerie said in a statement. “However, he has spent little effort making the case for his own candidacy to grass-roots movement conservatives. The first great challenge facing conservatives is whether or not Mitt Romney can heal the wounds created by his negative campaigning.”

Viguerie and other conservative leaders who had been backing Santorum met with the Pennsylvania Senator to plot strategy last week, convinced that his candidacy remained viable despite Romney’s delegate advantage. Now that Santorum has put his campaign on hold, conservatives appear reluctant to embrace Romney too quickly, determined to ensure that the presumptive nominee does not take them for granted.

A key question for the former Massachusetts governor is whether he can capture not just the endorsement but the active support of conservative organizers and voters.

Despite his frontrunner status and substantial Wall Street backing, Romney has failed to ignite enthusiasm among the conservative grass roots and has struggled to raise money from low-dollar donors.

Romney began the process of wooing conservative organizers even before Santorum dropped out.

As early as Monday, Romney’s campaign contacted Catholic Vote, an advocacy group opposed to abortion that has spent about $200,000 on ads and get-out-the-vote activities on Santorum’s behalf. The Romney camp signaled respectful interest in a conversation should Santorum decide to leave, Catholic Vote President Brian Burch said.

“I think it would be a mistake for us to simply jump in and say, ‘Well, of course we will support Mitt Romney,’” Burch said. “We aren’t just gadflies. We are an important voting bloc that expects to be taken seriously.”

Catholic Vote is one of more than a half-dozen conservative groups and super PACs that have spent just under $7.8 million on pro-Santorum campaign ads and expenditures, according to the Center for Responsive Politics. The bulk of that money came from the Red, White and Blue Fund, a pro-Santorum super PAC whose top backer is conservative businessman Foster Friess. Friess also spent $71,616 on pro-Santorum ads independent from the PAC. He has now signaled that he will back Romney.

Red, White and Blue Fund founder Nick Ryan issued a statement today that hailed Santorum as a “great leader” but did not endorse Romney or signal the super PAC’s plans going forward. Super PAC organizers could not be reached for comment.

“There is a lot at stake in November for our nation,” Ryan said in the statement. “We can do better than Barack Obama. But not only the presidency is at stake. We also have the opportunity to strengthen the conservative majority in the House of Representatives and to oust the liberal leadership in the Senate.”

Other conservative organizers who had backed Santorum with ads and organizing help were noncommittal today. Responding via e-mail, Eagle Forum executive director Colleen Holmes said the group “will certainly support whomever the Republican nominee is, although we don’t have specific plans just yet.”

Marjorie Dannenfelser, president of the anti-abortion Susan B. Anthony List, said in a statement that Santorum had “reached the hearts of pro-life voters and allowed them to show the strength of their voting bloc,” but she made no mention of Romney. The group spent more than $500,000 on a pro-Santorum campaign that included radio ads, voter mobilization and a multi-state bus tour.

There are signs that conservative leaders will eventually support Romney as the nominee. Conservative activist Gary Bauer, whose Campaign for Working Families had backed Santorum, “will work to defeat Barack Obama,” said his spokeswoman, Kristi Hamrick. “That’s what Gary will do.”

But Romney, who has struggled to deliver a consistent message on such issues as abortion and health care, is just at the beginning of a potentially awkward courtship.

“Everyone’s got to get comfortable,” Burch said. “Conservatives have learned that we can’t simply trust rhetoric. We almost have to take a measure of a man’s heart and his soul, not just his words. And that’s not something you just decide on a whim.”

Viguerie was more blunt.

“The next step is up to Mitt Romney,” his statement concluded. “Romney is seriously behind with committed conservative voters. To catch up, he must make the case that he merits the support of movement conservatives and that a Romney administration, if elected, can and will produce conservative government.”