Washington (CNN) - Former Mississippi Gov. Haley Barbour has no plans to formally endorse Mitt Romney now that his fiercest challenger, Rick Santorum, is no longer in the presidential race.
But Barbour, a respected Republican party elder who voted for Newt Gingrich in last month's Mississippi primary, acknowledged that Romney is the de facto GOP nominee and said he plans to help him get elected in November through his work with the powerful super PAC American Crossroads.
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"I didn't endorse anybody in 2008, and I am not going to endorse anybody this time," Barbour told CNN in a phone interview Wednesday. "But obviously I am going to do everything I can do to help Romney get elected."
Barbour is raising money for Crossroads, the well-funded super PAC co-founded by Karl Rove that recently began airing television ads attacking Barack Obama in key battleground states.
Crossroads must operate independently of any political campaign, the main reason Barbour said he isn't coming out with a full-throated endorsement of Romney.
"Because of my activity with American Crossroads, it's probably just as well I not allow any appearance that I am involved with Mitt's campaign," he said. "But I am very involved with American Crossroads in hopes of getting a new president named Mitt Romney."
Following a lengthy Republican primary fight, which raised questions about Romney's support among conservative voters and forced him to tack to the right on several key issues, Barbour advised Romney to aggressively focus on a policy-heavy message that questions Obama's handling of the economy.
Barbour said that theme will be enough to keep the conservative base excited while also appealing to independent voters who will ultimately decide the election.
"This campaign is going to be waged in the center," Barbour said.
"Don't ever forget that Barack Obama is the great uniter of Republicans. The party apparatus, the conservatives, the tea party, the organizations like small businesses and the NRA, they will be very active in providing volunteers and our base will be very active. That's critically important. But the election is going to be decided by a few million people, most of whom voted for Obama last time but have the same level of buyer's remorse as ticket purchasers on the Titanic."
Barbour also advised Romney to ignore the vice presidential chatter bubbling up around Washington now that the Republican race is effectively over.
"If I were giving advice about that, it would be don't hurry," he said. "Give a lot of time to make this decision and don't make it until you have to, because you have more good information three months from now than three weeks from now."
"I wouldn't even talk about it now except to say that our candidate needs to pick a running mate who is ready to be president," Barbour added.