ABC News' Michael Falcone reports:

Channeling country music legend Kenny Rogers, former President George H.W. Bush officially endorsed Mitt Romney in Houston on Thursday, advising Romney's remaining opponents that they've got to know "when to fold 'em."

"I do think it's time for the party to get behind Gov. Romney. And she was reminding me Kenny Rogers sang, 'It's time when to hold 'em and time when to fold 'em,'" Bush said, loosely quoting a lyric from the famous song, "The Gambler."  Well I think it's time for people to all get behind this good man. And, some of 'em waged a very good fight - I say that about some of his opponents. But we're so convinced and we've known Mitt for a very long time, that he's the man to do this job and get on and win the presidency."

Bush had already expressed support for Romney last December, telling the Houston Chronicle at the time that the former Massachusetts governor was "the best choice."

"Barbara and I are very proud to fully and enthusiastically endorse and support our old friend Mitt Romney," the former president said, flanked by Romney and former First Lady Barbara Bush. "He's a good man, he'll make a great president."

Romney, who used the trip to Texas to raise money, accepted the endorsement.

"I have a big day coming up next Tuesday with elections in Wisconsin, Maryland and the District of Columbia. I hope to get a good number of delegates from those three contests and be on my way to securing the nomination and taking on the president," Romney said. "With your endorsement I'll get a good boost from those contests and also here in Texas. I'm told they know you guys pretty well here."

Looking ahead to the Lone Star State's late May primary, Bush responded, "Been here a long time. You'll do well here, I'm convinced of that."

During the meeting, Romney was asked whether he had met with Bush's son, former President George W. Bush, who has not yet endorsed in the Republican primary.

"I haven't met with President George W. Bush," Romney said. "We speak from time to time."

Barbara Bush interjected: "We speak all the time."

At one point, Bush, who wore a white turtleneck, blue blazer and multi-colored socks (he gave a pair to Romney as a gift, according to the candidate's personal aide) asked his wife whether she had anything to add.

"Aren't you going to sing a song for us? Know when to fold 'em?" she said, prompting the musical interlude. (Bush said the line, but did not sing it).

Rick Santorum's spokesman, Hogan Gidley, saw Bush's "Gambler" quote and raised him another one.

"We have huge respect for President George H.W. Bush and he is one of my personal political heroes," Gidley said, "but my favorite line from that song is 'there'll be time enough for countin' when the dealin's done."