Obama campaign manager Jim Messina said in a statement after Santorum's withdrawal: "It's no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads. But neither he nor his special interest allies will be able to buy the presidency with their negative attacks. The more the American people see of Mitt Romney, the less they like him."
Other Obama campaign officials have mocked Romney's wealth and called him out of touch with average Americans.
Romney and his allies, including a potent super political action committee, have proved their ability to raise millions of dollars to air brutally effective attack ads, which crippled Santorum and Newt Gingrich in the GOP primary contests. Obama will raise many millions, too, and few doubt that he will hit Romney hard.
The Republican super PAC Crossroads GPS is already on the air attacking Obama in six critical swing states. The group is spending $1.7 million to attack the president's energy record for a week in Colorado, Florida, Iowa, Nevada, Ohio and Virginia.
On Tuesday, Romney made clear that he will go after Obama's character as well as his record. In speeches in Mendenhall and Wilmington, Del., Romney said Obama isn't merely inept at economic policy, he actively dislikes business.
Obama "is clearly trying to hide from us what he intends to do," Romney said in Wilmington. "He's going to hide. And it's my job to seek."
Romney made similar remarks last month. Now, with Santorum off the stage and Gingrich and Ron Paul hardly a factor, there are no intra-party distractions to dilute such comments. Romney and Obama are fully engaged, one on one, at a much earlier stage than in 2008, when Obama had to parry Democratic rival Hillary Rodham Clinton throughout the summer before fully turning to Republican John McCain.
Even then, Bush's unpopularity helped fuel Obama's campaign and deflected some of the anti-GOP sentiment away from the actual nominee.
This time, the incumbent president is on the ballot, with unemployment above 8 percent. The tea party, which didn't exist in 2008, is a potent and unpredictable force.
And Romney suddenly is free of meaningful primary worries. That leaves him able to focus the full force of his fundraising and campaigning skills against Obama.
Associated Press writer Kasie Hunt in Washington contributed to this report.
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