Why hasn't Rick Santorum endorsed Mitt Romney yet?

Rick Santorum dropped out of the presidential race eight days ago, but he has yet to endorse the presumptive Republican nominee, Mitt Romney. What’s more, a Santorum fundraising letter arrived in Iowa voters’ mailboxes on Monday that was highly critical of Mr. Romney.

And in a conference call with more than 4,000 supporters Monday night, Mr. Santorum was asked by the host – reportedly in jest – if he would “unsuspend” his campaign if enough people voted for him in next Tuesday’s primary in Pennsylvania, his home state.

“I would just say this,” Santorum replied, according to The National Journal. “The best thing that they can do is stay tuned, and we really are serious about making sure that the issues that we brought up during this campaign are continued going forward.”

Amid a divisive primary season, in which many Republicans have raised concerns about Romney’s conservative bona fides, should the former Massachusetts governor worry about Santorum?

Not really. There’s often a lag between a candidate’s dropout announcement and his or her endorsement for president. Four years ago, Romney took a week after dropping out to endorse the eventual nominee, John McCain. The eventual entente involved delicate negotiations between the two camps, and was facilitated by personal relationships between aides to both men, according to published reports at the time.

As part of the deal, Romney released his convention delegates to Senator McCain. But on Monday, Santorum raised eyebrows when he suggested he may not release his delegates even if he backs another candidate.

“We still have delegates, many of them committed, and we want to make sure that our delegates get a chance to go to the convention and have a say,” Santorum told his supporters.

Santorum and Romney are going to get together “in the next month or so” and “have a conversation,” Hogan Gidley, Santorum’s communications director, said Wednesday on MSNBC. “I do know for a fact an endorsement will be one of the topics discussed. But of course there’s no guarantee that an endorsement comes out of that meeting. It’s just a conversation they’re going to have.”

Santorum does have some leverage with Romney: He won 11 states and more than 3 million votes in the primaries. Romney needs Republicans to rally around him by donating, volunteering, and ultimately, voting for him. Polls show that in fact, 90 percent of Republicans already say they’re prepared to vote for Romney – the same percentage of Democrats who say they’ll support Obama. So for both men, it’s a question of turning out every last vote, particularly in battleground states, in what is expected to be a tight race.

On the deficit side of Santorum’s ledger, he has almost $1 million in campaign debt, and is eager to retire it. In his call with supporters, he asked for donations. On Tuesday, he sent out a solicitation email.

Does Santorum want Romney to help retire his debt? John Brabender, a top Santorum adviser, was asked on MSNBC Wednesday.

“I think you will see Rick Santorum as an active participant campaigning this fall, but ... there are some things that have to happen first, to make sure that he’s wanted by the Romney campaign, and we’re going to talk about the issues that conservatives think are very important,” Mr. Brabender replied.

One GOP strategist not involved with either campaign believes Santorum has earned the right to drag out his endorsement – and is very likely looking for both a role at the Republican convention and help retiring his campaign debt.

“I wouldn't say the grace period is over," says the strategist, Ford O’Connell, who thinks Santorum has till June to endorse. “They’re smartly working the Romney camp.”

Santorum is also working out his next steps as a conservative leader – including putting together "a structure" to promote his ideas – and promised on his call with supporters that he would make an announcement in the next two weeks.

And what about those fundraising letters that landed in Iowans’ mailboxes on Monday? The Santorum campaign says those went out before he decided to suspend his campaign.