LIBERTY, Mo. (AP) -- Mitt Romney isn't waiting to lock down the GOP presidential nomination to begin focusing his campaign chiefly on President Barack Obama.
The trend is born of strategy and necessity. The more Romney portrays himself as a formidable challenger to Obama, the more he may be able to persuade lukewarm Republican voters he's the most "electable" candidate.
"I'm the one guy in this race who can beat Barack Obama," he told 400 sun-drenched people Tuesday in Liberty, Mo. Meanwhile, the clock is ticking towards November, and Romney is loath to wait any longer to start denting Obama's armor.
But his inability to pull cleanly away from former Sen. Rick Santorum is forcing Romney to spend precious time and money in states he'd prefer to ignore. He made several campaign stops in Alabama and Mississippi, which are solidly in the GOP column in November.
He will spend much of the weekend in Puerto Rico, another place that doesn't factor into general election strategies. And the chief pro-Romney super PAC is buying ads in Louisiana, which fits the same category.
The former Massachusetts governor is doing his best to pretend Santorum and his fellow rivals Newt Gingrich and Ron Paul hardly exist. Romney ignored them Tuesday at his two public events, in Missouri, which holds caucuses Saturday. Instead he ripped into Obama on health care, gasoline prices, foreign policy, the deficit and other issues.
"I wish, as a president, he would finally take responsibility," Romney told an outdoor crowd in the Kirkwood suburb of St. Louis. "Take responsibility for 24 million people out of work or underemployed in this country. Take responsibility for not having drilled for more oil when he could have. Take responsibility for home values having gone down, down, down."
In Liberty, Romney said: "The president has failed. He's not a bad guy. He's just over his head."
On Monday, Romney briefly referenced Santorum and Gingrich without naming them. "A couple of guys" running against him have spent their careers in Washington, Romney told a crowd in Mobile, Ala.
He said his background in business, the Winter Olympics and Massachusetts makes him better able to attack the nation's problems.
Romney is willing to criticize Santorum and Gingrich, sometimes forcefully, when they come up in radio and TV interviews.
CNN asked Romney on Tuesday about an attack ad by Santorum's allies. Romney said the independent group FactCheck has examined the claims and "in almost every case, they've said that Rick Santorum's attacks have been baseless and wrong."
"Sen. Santorum is at the desperate end of his campaign," Romney said.
Romney, however, can go for long stretches in public appearances with barely a mention of Santorum, Gingrich and Paul.
His allies say Romney's steady collection of GOP delegates makes it almost impossible for his rivals to catch him. But revised party rules, which allow many states to allocate delegates proportionately instead of winner-take-all, make it difficult for Romney to secure the 1,144 he needs before mid-summer, if then.
"This is all about getting delegates," Romney told reporters Tuesday as he awaited the Alabama and Mississippi results.
He made the comments in Missouri, which underscored his dilemma. Obama campaigned hard in Missouri in 2008 but narrowly lost, and his allies don't see it as a high priority this year. Romney surely would prefer to spend more time focused on Florida, Ohio, Iowa or the other tossup states that will decide the November election.