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. The Democrat already has spent weeks hammering the Republican on his record at the private equity company he founded.

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.

He's also picking up high-profile endorsements. Former Secretary of State Condoleezza Rice confirmed to The Associated Press that she is backing Romney.

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.

It is a sign of the overarching argument Romney will make against Obama in the coming weeks: that Obama's economic efforts have been politically motivated, have wasted taxpayer dollars and, ultimately, have failed.

The renewed Solyndra criticism is also an attempt to answer Obama's recent criticism - in speeches and advertisements - of Romney's tenure at the helm of Bain Capital. Obama has argued that Romney's time as a private equity executive before entering politics - the basis for his campaign's claim that he's best suited to lead an economic recovery - was aimed at enriching shareholders, not serving the public.

Romney largely survived that opening salvo, judging by early polls and interviews with both Republicans and Democrats.

National surveys show Obama either statistically tied with Romney or only slightly ahead of him. And polls in the most hotly contested states show Romney competitive with Obama.

"They haven't worked," GOP strategist Charlie Black, an informal Romney adviser, said of the Bain criticisms. "People are worried about the economy and jobs."

Democratic strategist Steve Elmendorf said too few voters had heard the argument.

"The reality is it's June," said Elmendorf. "To say the Bain argument has worked or not is ridiculously premature."

A handful of swing-voting states will be the most hard fought as Obama and Romney aggressively build get-out-the-vote organizations and air a flood of advertisements in their efforts to reach the 270 Electoral College votes needed to win the White House.

Across the country, Romney's campaign is going through growing pains as it seeks to turn a stripped-down primary operation into a full-scale general election campaign. Aides say offices will open and more staff will join the team, but they also say they don't anticipate being able to match the staffing levels of Obama, who has had teams in place for months in states like Florida and Ohio. The Republican National Committee also is in the midst of boosting its staffing to help Romney.

Already, Obama's and Romney's campaigns, as well as super PACs supporting and opposing each, have spent more than $80 million in advertising on the general election campaign through the month of June, according to ad-tracking reports provided to The Associated Press by Smart Media Group of Alexandria, Va.

Of that, Obama's campaign has spent $31 million-plus on TV ads in about 10 states. He's been on the air most heavily in Florida, Iowa, Ohio and Virginia, an indication of where his team may think the race will be won or lost. A Democratic-leaning super PAC also has been on the air to help him, though it's spent only about $6.3 million.

Romney himself has run ads in just five battleground states: Iowa, New Hampshire, North Carolina, Ohio and Virginia. But he's been significantly helped by pro-Romney outside groups, who have been on the air in several others - most heavily in Florida, Ohio and Virginia.

Even so, Republican outfits, including Romney's campaign and supporting super PACS, have combined to spend about as much as Obama and his allies. And in the coming weeks, Romney plans to focus heavily on fundraising so that he'll be able to make strategic decisions about where to devote most of it state by state.

To that end, Romney is spending the week courting donors in California, starting Wednesday in Fresno, Bakersfield and in Hillsborough, a wealthy enclave just outside San Francisco.

He planned major fundraising events Thursday, Friday and Saturday in southern California. On Tuesday, Romney was in Las Vegas wooing Sheldon Adelson, the primary financing source of a group that backed Newt Gingrich's failed primary campaign. And Romney also attended a fundraiser hosted by Donald Trump.

Associated Press writers Kasie Hunt and Steve Peoples in California, Beth Fouhy in New York and Anne Gearan in Washington contributed to this report.