Rick Santorum finally endorsed Mitt Romney for president, but he sure didn't trumpet the fact.

The word came near the end of the 13th paragraph of an e-mail that hit the inboxes of Santorum supporters about 11 p.m. Monday - more than 900 words into his 1,107-word message.

The former Pennsylvania senator wrote of his "clear differences" with Romney, illuminated during their bitter fight in the primaries, but said he was reassured that the presumptive Republican nominee would stand up for conservative principles after a private meeting the two men held last week in Pittsburgh.

"Above all else, we both agree that President Obama must be defeated," Santorum wrote. "The task will not be easy. It will require all hands on deck if our nominee is to be victorious. Gov. Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime."

The seemingly offhand endorsement - with no traditional smiling photo op - caused political analysts to question whether Santorum harbors reservations about Romney's conservative bona fides - and, more important, whether that signals a broader coolness toward him among religious and other social conservatives in the GOP base.

"Sen. Santorum ran a spirited race and his commitment to conservatism energized millions of Republicans around the country," Romney said in a statement. "The race for the Republican presidential nomination has always been about restoring the promise of America. Sen. Santorum and I share an absolute commitment to that goal."

Some pundits suggested that Santorum had maneuvered himself out of a prime role at the GOP national convention in Tampa, Fla., this summer with his faint praise for Romney.

"The notion that Santorum owns some constituency is, you know - many other conservatives can speak at the convention that will rally the conservative base in a way that's probably greater than Rick Santorum could," Republican strategist Mark McKinnon said on MSNBC's Morning Joe. "So there's no downside to just tell him to take a hike."

Joe Scarborough, the former Republican congressman from Florida who hosts the program, called the endorsement "bush league" and said Santorum should be relegated to speaking "in that cage where the protesters are going to be, 11 blocks away" from the convention hall.

Santorum was the final and strongest of a series of conservative alternatives who rose up to challenge Romney in the GOP primaries until he folded his campaign April 10, in debt and facing potential defeat in the Pennsylvania primary. He had won 11 states and collected more than three million votes.

In his note to supporters, he made it clear that he would remain a leader in the conservative movement and said that he wanted to work with Romney to ensure that the presumptive GOP nominee did not waver. He also said he would be disclosing more about his plans soon.

"During our meeting I felt a deep responsibility to assess Gov. Romney's commitment to addressing the issues most important to conservatives, as well his commitment to ensuring our appropriate representation in a Romney administration," Santorum wrote. Romney, he added at one point, "listened very carefully to my advice."

The two met alone in Pittsburgh on Friday in the office of Santorum's top political adviser, John Brabender. Though Romney had a rally scheduled at a nearby cement factory after the meeting, they did not appear together. Santorum aides said an endorsement was not discussed.

Brabender did not respond to a request for comment Tuesday.

Jeff Coleman, a GOP consultant in Harrisburg who backed Santorum, said the tone of the e-mailed endorsement was fine, considering the stakes for the social-conservative grassroots.

"Rick has a new national constituency, and that was built on an authentic voice," Coleman said. He said Santorum laid out "performance measures" on behalf of the conservative base "to get some of the big issues locked down."

Former U.S. Rep. Phil English, a Republican from Erie who was heading Romney's primary campaign in the state, said Tuesday that he did not believe conservative enthusiasm would be a problem.

"I don't feel there's any division in the party," English said. "We have a clear outcome to the nominating process, and I don't think most Republicans view this with as many qualifiers as Rick Santorum does."

English said it is "entirely possible" that Romney will include Santorum in the traditional convention lineup of speeches from former rivals, but "there's nothing in the election results that obligates the national Republican Party to make him the centerpiece of the convention."

Recent polls suggest Republicans are rallying around Romney. A Pew Research survey in April found that 90 percent of Republicans who identify themselves as conservatives plan to vote for the former Massachusetts governor if he is the nominee, compared with 84 percent of moderate GOP voters.

"Committed social conservatives will definitely vote against Obama," Coleman said. "The question is, do they fill the car with cousins and friends who are not Republican super-voters and bring them along to the polls? That's where the margin lives."