HOUSTON (Reuters) - Step by step, Mitt Romney is tightening his grip on the Republican presidential nomination race despite a continued penchant for gaffes on the campaign trail.
Romney was in Houston on Thursday to accept the endorsement of former President George H.W. Bush, the latest in a line of establishment figures to choose the former Massachusetts governor as the Republicans' best chance of defeating Democratic President Barack Obama in the November 6 election.
The endorsement came a day after Romney gained the seal of approval from U.S. Senator Marco Rubio of Florida, a hero of Tea Party conservatives who could help Romney improve his shaky ties with the most conservative members of the right wing of the party.
It also followed Romney's latest campaign stumble - in which he told Wisconsin voters on a conference call what Romney thought was a funny story about his father, former Michigan governor and auto executive George Romney, closing a factory in Michigan and moving the jobs to Wisconsin.
Democrats leaped on the incident as another sign that Romney, a former private equity executive with a personal fortune of $190 million to $250 million, is detached from the concerns of most Americans.
Romney has committed several similar gaffes recently, including telling voters in economically struggling Michigan that his wife, Ann, drives two luxury Cadillacs.
But Romney has weathered each gaffe and survived challenges from top rivals Rick Santorum and Newt Gingrich to take a commanding lead in the race for the 1,144 party delegates needed to clinch the Republican nomination.
Romney is about halfway to that total; his delegate count is nearly double that of Santorum's, his closest challenger.
Romney seems poised for a good showing in the next round of contests on Tuesday in the state-by-state battle to pick a nominee. Polls show he leads in all three - Wisconsin, Maryland and Washington, D.C.
A sweep would put more pressure on Santorum, a former Pennsylvania senator, to give up his fight for the nomination. Santorum has vowed to stay in the race despite his increasingly slim chances of catching Romney in the race for delegates.
A defeat in Wisconsin, a Midwestern state that will be a key battleground in the general election and where Santorum led in polls a few weeks ago, would be particularly damaging to Santorum.
As in other states, Romney is heavily outspending Santorum in Wisconsin.
Romney's campaign had spent more than $700,000 there as of Thursday, and the "Super PAC" supporting him has spent about $2.3 million on ads in Wisconsin, while Santorum's campaign and the Super PAC that backs him had spent a total of about $400,000.
Gingrich, a former U.S. House of Representatives speaker, also has promised to stay in the race until Romney has secured his 1,144th delegate. However, Gingrich's campaign is low on cash, and this week he trimmed his staff and cut back his schedule.
Now, the Las Vegas casino magnate whose huge donations essentially have helped to keep Gingrich in the race is portraying Gingrich's campaign as over.
"It appears that he's at the end of his line," Sheldon Adelson said of Gingrich in a video posted online Wednesday by the Jewish Journal. "Mathematically he can't get anywhere near the (needed delegate) numbers, and it is unlikely to be a brokered convention."
Winning Our Future, the Super PAC that has boosted Gingrich's campaign, has raised just over $18 million; more than $16 million of that has come from Adelson's family.
A Republican source said Gingrich met with Romney on Friday, but it is unclear what they discussed.
At Bush's office in Houston on Thursday afternoon, the former president urged Republicans to back Romney. Joined by his wife, Barbara, and Romney, Bush borrowed a line from a song about poker by country singer Kenny Rogers.
"You got to know when to hold 'em, know when to fold 'em," Bush said. "It's time for people to get behind this good man. ... He'll make a great president."
Romney also has been endorsed by Jeb Bush, a son of the former president and a former Florida governor. Former President George W. Bush, another Bush son, has said he will not make an endorsement.
The long list of Republican leaders endorsing Romney has not been enough to persuade some conservatives to support him.
However, Rubio's backing could help ease lingering doubts about Romney among some conservatives who distrust him for taking moderate positions on a range of issues in the past, including his support of a healthcare plan in Massachusetts that became a model for Obama's federal overhaul.
"Senator Rubio, who is very popular with conservatives, really sends a strong signal," Republican strategist Doug Heye said.
Santorum has run as an insurgent underdog, tapping into conservative anger about Washington. But Santorum's comment this week that he would consider the vice presidential nomination if his campaign falls short could undercut his support.
"That's not an argument that someone who is fighting for the nomination makes," Heye said.
Romney has been prone to gaffes that remind voters of his vast wealth, and the Wisconsin story fit the pattern. He recalled how his father, as head of American Motors Corp., closed a factory in Michigan and moved all production to two factories in Wisconsin.
The move later became a sensitive issue in George Romney's campaign for governor in Michigan. In one embarrassing episode, Romney said, his father was part of a parade in which the marching band played one song: "On Wisconsin."
Chuckling, Romney said, "Every time they would start playing 'On Wisconsin, on Wisconsin,' my dad's political people would jump up and down trying to get them to stop because they didn't want people in Michigan to be reminded that my dad had moved production to Wisconsin."
Additional reporting by Alina Selyukh, Alexander Cohen and Lily Kuo in Washington and Deborah Quinn Hensel in Houston; Editing by David Lindsey and Eric Beech