WASHINGTON/MILWAUKEE President Barack Obama attacked Mitt Romney directly on Tuesday for backing a controversial Republican budget plan, marking a shift into the general election as the former Massachusetts governor edged toward becoming his party's presidential nominee.
Obama rarely mentions the Republican front-runner by name, preferring to let Romney battle it out with party rival Rick Santorum, a former Pennsylvania senator.
Romney is moving closer to taking on the Republican presidential mantle and is expected to have well over half of the Republican delegates needed to win after primary votes in Wisconsin, Maryland, and Washington, D.C. on Tuesday.
So the Obama campaign has begun targeting him more overtly, and the president rounded on Romney, a wealthy former private-equity executive, for supporting the Republican budget plan that would make stark cuts in Medicare and other programs for the poor and elderly.
"One of my potential opponents, Governor Romney, has said that he hoped a similar version of this plan from last year would be introduced as a bill on day one of his presidency," Obama said in a speech to news executives.
"He said that he's very supportive of this new budget and he even called it 'marvelous', which is a word you don't often hear when it comes to describing a budget."
In the midst of a lackluster U.S. economic recovery, issues such as the deficit, high unemployment and the rising cost of gasoline are likely to dominate the November 6 general election.
Obama has made economic fairness a key component of his speeches for months, and his campaign released an ad targeting Romney for supporting oil companies, which Romney promptly ridiculed - focusing his criticism squarely on the president, rather than his Republican rivals. The back-and-forth set up the framework for a general-election fight.
"So the president put an ad out yesterday, talking about gasoline prices and how high they are. And guess who he blamed? Me!" Romney said in Waukesha, Wisconsin.
"Maybe after I'm president I can take responsibility for things I might have done wrong. But this president doesn't want to take responsibility for his mistakes."
A sweep of all three of Tuesday's contests would underscore Romney's growing strength and likely increase appeals from party leaders for Republicans to rally behind him, despite deep reservations among many conservatives suspicious about whether he is one of them.
Polling closes in Wisconsin at 9 p.m. EDT (0100 GMT, Wednesday) and in the two other primaries at 8 p.m. EDT.
Practically, winning all three contests could give Romney 98 more delegates, putting him well over half of the 1,144 needed to clinch the nomination at the Republican convention in August.
And it would set the tone for the next big date on the campaign calendar, April 24, when six states hold Republican presidential contests. Romney leads in five of them and plans to make an aggressive push in the sixth, Santorum's home state of Pennsylvania. A Quinnipiac University poll released on Tuesday showed Santorum ahead of Romney there by 41 to 35 percent.
A Gallup Daily tracking poll found Romney ahead nationally over Santorum by 16 points as Republican voters' pick to be the party's nominee.
Sensing the nomination is in sight, Romney has made no mention in recent days of Santorum or his other Republican rivals, Newt Gingrich and Ron Paul.
His campaign also plans to start raising money jointly with the Republican National Committee, another sign of a shift to a general-election focus.
Although Romney has locked up support from much of the Republican-party establishment, he has struggled to win over strict conservatives, many of whom favor Santorum.
Sarah Palin, the conservative former governor of Alaska and 2008 Republican vice-presidential nominee, said the nominating process could still veer away from the front-runner.
"Anything is still possible, there can still be a bit of a shakeup," she said on NBC's "Today" program.
Palin said whoever becomes the nominee - Romney or one of his rivals - would get the support of the Republican base.
"He (Romney) will be able to do that," she said. "If not Romney, if one of the other GOP candidates happen to surpass Romney in the delegate count ... whomever will be able to coalesce, we will be able to coalesce around that nominee and make sure that voters understand they have a choice here."
Wisconsin is the most closely watched race of the trio voting on Tuesday. Santorum, known for his sharply conservative views, has campaigned heavily in the state and led in the polls until ceding the lead to the Romney in the last week or so.
If he does win the nomination, Romney would face the challenge of defeating an incumbent president whose campaign operation is well-funded, organized and eager to pounce on any misstep.
Santorum aims to survive Wisconsin and the rest of April and move on to May, when the states that vote may be more favorable to him.
He would have to win an overwhelming percentage of the remaining delegates to win the nomination outright.
But Santorum seems to have a different strategy: Win enough delegates to deny outright victory to Romney. This would force Republicans to choose their candidate at a "brokered" convention in Tampa, a chaotic scenario that many political experts believe could be disastrous to the party's hopes of ousting Obama.
"I would argue even if it ends up in a convention, that's a positive thing for the Republican Party, that's a positive thing for activating and energizing our folks heading into this fall election," Santorum told reporters on Monday.
Trying to appeal to blue-collar voters, Santorum has held small campaign events in Wisconsin, frequently appearing in bowling alleys. He insists he is staying in the race and has been relentless in trying to brand Romney as a Massachusetts moderate who would govern little differently than Obama.
(Additional reporting by Sam Youngman and Deborah Zabarenko; Editing by David Brunnstrom)
(This story corrects the number of delegates at stake in paragraph 13)