Mitt Romney is mobilizing a fast-growing network of surrogates to help make his case with voters as his campaign begins to exert greater control over the GOP messaging operation.

Romney is relying on a diverse cast of politicians, business leaders, athletes and celebrities to court key groups of voters, including social conservatives, Hispanics and suburban women.

The campaign is dispatching designated hitters across the country to speak at dinners and fundraisers — Sen. John Thune (S.D.), former Minnesota governor Tim Pawlenty and former ambassador John Bolton are working that circuit. Other surrogates, such as former New Hampshire governor John Sununu, whom Romney recently called his “bulldog,” serve up scathing soundbites about the Democrats.

“With certainty being brought into the process the last few months, we’re seeing a flood of people who want to get involved in the campaign,” said Kristy Campbell, the campaign’s deputy communications director

It is politically risky for Romney to hitch his fortunes to other people; he was burned this week when surrogates veered off script. But the former Massachusetts governor and his advisers have calculated that, without a strong natural base constituency, he needs others to help him make the sale.

At Romney’s Boston headquarters, aides are trying to build a more disciplined surrogate operation, distributing talking points to politicians and pundits whom they call upon to spread his message. Romney aides are now picking guests to appear on the Sunday political talk shows and holding Saturday conference calls to rehearse answers to likely questions, according to a campaign adviser.

Other surrogates are booked for targeted television, radio and newspaper interviews to help build support among demographic groups with which Romney has struggled.

“That casserole is being baked now,” said longtime GOP strategist Ralph Reed. “The Romney campaign is determining who can best appeal to which audiences.”

Romney’s advisers consider the candidate’s wife, Ann, to be his most powerful surrogate and are developing a robust schedule of solo visits for her to help close the gender gap with Obama. Ann Romney plans an expanded media presence and this week hired her own press secretary, Sarah Haley.

Believing she showcases her husband’s softer side to moderate voters, an adviser said they are considering having her campaign in nursing homes, schools and medical research facilities in suburban areas outside Philadelphia, Denver, Milwaukee and Charlotte, as well as in Northern Virginia and along Florida’s I-4 corridor.

Other female surrogates are making similar pitches, including Sen. Kelly Ayotte (N.H.), South Carolina Gov. Nikki Haley and Rep. Cathy McMorris Rodgers (Wash.).

This comes amid increasing speculation about Romney’s ultimate surrogate: his vice presidential running mate. Romney is road-testing the contenders with a series of joint campaign appearances, and they are doing solo interviews and speeches on his behalf.

So far, Romney’s top surrogate might be New Jersey Gov. Chris Christie, who joined Romney at a $5 million fundraiser in New York last week.

“He is the fullback you keep running into the line for four or five yards at a time, and you keep running him until he can’t run anymore,” said Daniel Schnur, a veteran of past GOP presidential campaigns.

Romney is turning to supporters who have credibility with particular groups to vouch for him. For instance, Sen. Ronald H. Johnson (Wis.), a freshman aligned with the tea party, has been calling in to conservative talk radio shows for Romney.

If Wisconsin Gov. Scott Walker survives next week’s recall election, Schnur said, he will become an “extremely valuable” surrogate, as is Rep. Paul Ryan (Wis.). “These are all people who carry more currency with conservative voters than Romney himself,” he said.

The same could be said for some of Romney’s rivals from the primaries, including former House speaker Newt Gingrich and former senator Rick Santorum. Gingrich has surprised some Romney allies with his strong defense of Romney in recent appearances, and plans are underway for Santorum to stump in battleground states to help rally conservatives behind Romney.

Campbell would not discuss Santorum’s plans, but a Republican close to Santorum said he intends to campaign for Romney in parts of the industrial Midwest, where he connected with social conservatives and white working-class voters in the primaries.

Romney saw this week the damage some off-message surrogates can cause. As he clinched the nomination on Tuesday night, he shared the stage at a Las Vegas fundraiser with real estate mogul Donald Trump, who had spent the day reasserting his debunked belief that President Obama was born outside the United States.

Last Sunday, Rudy Giuliani went on CNN’s “State of the Union” as a Romney surrogate only to give a back-handed endorsement by saying his job creation record as mayor of New York was stronger than Romney’s as governor of Massachusetts.

Such slip-ups are more the exception than the rule, however, and Romney believes it’s a risk worth taking.

I don’t agree with all the people who support me, and my guess is that they don’t all agree with everything I believe in, but I need to get 50.1 percent or more, and I’m appreciative to have the help of a lot of good people,” Romney told reporters this week.

The Romney campaign is trying to spotlight non-political surrogates as well. It booked Staples founder Tom Stemberg, who worked with Romney at Bain Capital, on CNBC Wednesday morning. Actor Jon Voight, NFL player Nick Mangold, comedian Jeff Foxworthy and singer Kid Rock have campaigned with Romney. And endorsements from Olympic champion speed skaters Bonnie Blair and Derek Parra play at the beginning of Romney’s tele-town hall conference calls with voters.

The campaign is enlisting Latino supporters to give Spanish-language interviews and visit community events in states including Florida, Nevada and Colorado.