DES MOINES, Iowa – What’s next for Mitt Romney? The Republican presidential nomination finally in hand, he will spend the next three months trying to undercut President Barack Obama on the economy while portraying himself as Mr. Fix It for a nation with stubbornly and painfully high unemployment.

Romney also faces key decisions between now and his acceptance of the party’s nomination in late August in Florida: Where should he compete most aggressively? Who should be his running mate?

At the same time, he must dive anew into fundraising and work to win over voters who are distracted by their own summer plans and day-to-day pocketbook worries – while withstanding Obama’s attacks on his own claims as a jobs creator.

Not that Romney is publicly sweating the hurdles that come with being the little-known challenger to a personally popular president.

“People will get to know me better,” Romney told Fox News in an interview that aired Wednesday, the day after he sealed the GOP nomination with his primary election victory in Texas. He says the general election campaign is only beginning even though his chief challenger dropped out more than six weeks ago.

With a smile, he said of the voters, “My guess is they’re going to get to know more about me than they’d like to by the time we’re finished.”

As if on cue, Obama’s campaign opened a fresh critique of the GOP nominee-in-waiting, assailing his economic record as governor of Massachusetts. It’s the second phase of an effort by Obama to define Romney negatively in voters’ eyes.

Obama himself made a courtesy call to Romney to congratulate him on his nomination victory. An Obama aide said the president told Romney in a brief and cordial chat that he looked forward to debating America’s future with him.

Romney spent the day in California, plunging into a week filled with fundraisers and efforts to unite Republicans after a divisive primary season. Already he’s proven adept at both, hauling in enough cash to cut into the advantage that Obama has while getting most of his former Republican rivals to close ranks around him.

Those efforts – and the turning of his primary campaign into a general election operation – have been his prime focus. He’s making only a handful of public appearances for now, but aides say they expect the campaign to ramp up to a full sprint by July 4. Romney has said he plans to take a week off around the holiday, suggesting that may be the time when he makes final deliberations on whom to choose as his vice presidential nominee.

Little is known about just where in that process Romney may be, though there is no shortage of Republican rising stars informally auditioning for the role.

While work on that front is certainly going on behind the scenes, Romney’s aides are spending this week publicly pressing anew a criticism that the candidate himself has been making for months against Obama.

They’re highlighting the hundreds of millions of dollars in economic stimulus money that the administration provided to Solyndra, the solar-energy company that went bankrupt and whose executives had contributed to Obama’s campaign.