(CNN)-- There was no press conference, no television cameras, no reporters and it didn't even arrive when the sun was up.
Instead, Rick Santorum's endorsement of presumptive Republican presidential candidate Mitt Romney came in paragraph 13 in an email to supporters at approximately 11 p.m. Monday night.
– Follow the Ticker on Twitter: @PoliticalTicker
"We both agree that President Obama must be defeated. The task will not be easy. It will require all hands on deck if our nominee is to be victorious," Santorum wrote. "Governor Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime."
With that, Santorum became the latest former rival to back Romney and the latest former rival to do so in a lukewarm fashion. Former House Speaker Newt Gingrich offered a far from ringing endorsement of Romney during a press conference last week, during which he painted the frontrunner as a positive alternative to President Barack Obama.
Santorum said he waited to make an endorsement until he could meet with Romney in person. The two met privately Friday in Pittsburgh.
"I felt that it was completely impossible for me to even consider an endorsement until after a meeting to discuss issues critical to those of us who often feel our voices are not heard by the establishment: social conservatives, tea-party supporters, lower and middle income working families," Santorum wrote.
However, he did not say whether he planned to join Romney on the stump before the November election.
Santorum enjoyed support from more conservative voting blocs as well as members those supportive of the grassroots tea party movement during the primary campaign, which helped him place first in 11 early contest states. In Monday's letter, Santorum said he encouraged Romney to "add more conservative leaders as an integral part of his team."
The former Pennsylvania senator said the contentious 2012 campaign proved he and the former Massachusetts governor "have some differences," but also areas in which they agree, including over the need for smaller government, opposition to abortion and the belief that marriage should be between one man and one woman.
One of the biggest gripes Santorum had on the campaign trail with Romney was the health care law he passed while governor of Massachusetts, but in Monday's note Santorum said he no longer doubts Romney's opposition to the current national health care law.
"While I had concerns about Governor Romney making a case as a candidate about fighting against Obamacare, I have no doubt if elected he will work with a Republican Congress to repeal it and replace it with a bottom up, patient, not government, driven system," Santorum wrote.
Romney expressed "sincere gratitude" for Santorum's endorsement and said his "commitment to conservatism energized millions of Republicans around the country," in a statement released Tuesday.
"Senator Santorum and I share an absolute commitment to that goal, just as we share an absolute commitment to reversing the failing policies of the Obama Administration, from its assault on freedom of conscience to its feckless foreign policy," Romney said.