MILWAUKEE >> Republican presidential front-runner Mitt Romney urged Republicans to shift focus to the general election, giving a subtle push to rival Rick Santorum as voters headed to polls today in Wisconsin, Maryland and the District of Columbia.

“The right thing for us, I think, is to get a nominee as soon as we can and be able to focus on Barack Obama,” Romney told Fox News in an interview. “You have to remember that it was Ross Perot that allowed Bill Clinton to win.”

Perot ran as an independent in the 1992 general election, when Clinton defeated President George H.W. Bush.

At the same time, Romney and President Barack Obama are trading jabs and acting as though the country’s biggest political matchup of the year is all but set. Obama’s re-election campaign is running a new TV ad in six swing states criticizing the former Massachusetts governor by name for the first time — in this case as a backer of “Big Oil” amid high gas prices.

While charging that Obama’s version of a perfect world is one with “a big-spending, big government,” Romney is behaving as though his three rivals for the nomination no longer matter.

“My campaign is going to continue focusing on Barack Obama,” Romney said today.

Romney scheduled one campaign event today before an election night party in Milwaukee. He spent the weekend campaigning across Wisconsin in the company of Rep. Paul Ryan, working to win yet another big industrial state that rival Santorum was counting on to keep his flagging candidacy alive.

Santorum was spending the day in Texas at private fundraisers for his campaign before heading to his home state of Pennsylvania for an election night party in Mars, just north of Pittsburgh.

Romney has 572 delegates to the Republican National Convention, half the needed 1,144, and is on a pace to clinch the nomination by the end of the primary season in June. Santorum has 272 delegates, Newt Gingrich 135 and Ron Paul 51.

There were 95 delegates at stake in today’s contests, including 42 in Wisconsin, the only one of the three contests Santorum has seriously contested. Romney is expected to do well in Maryland and in the District of Columbia, where Santorum is not on the ballot.

Romney has ignored Santorum the past few days to focus on Obama, telling supporters in Green Bay that the president “takes his political inspiration from the capitals of Europe.”

Obama’s ad claims that “Mitt Romney’s stood with Big Oil – for their tax breaks, attacking higher mileage standards and renewables. It’s in response to an ad from the American Energy Alliance blaming Obama for rising gas prices.

Romney’s campaign, though, is running far behind the president in fundraising, as he’s been unable to raise general election money because the primary contest is still going on.

At the end of February, Obama reported $84.7 million in his campaign account compared with Romney’s $7.3 million. Obama has more than 530 paid staff compared to roughly 100 for Romney.

But Romney has far outspent his rivals during the nomination fight.

Santorum, who also campaigned in Wisconsin on Monday, said Romney has essentially bought his success by outspending the competition.

Romney and his allies have spent $53 million on television advertising so far this election cycle compared to $27 million from his three Republican competitors combined, according to data compiled by the media tracking firm SMG Delta.

Santorum’s team, having narrowly lost a string of high-profile contests, spent just $9 million.

“With almost unlimited resources, Gov. Romney has not proven to be very effective,” Santorum said as he predicted a possible upset in Wisconsin. “The only way he’s been successful in winning the primaries is by just bludgeoning his opponents by an overwhelming money advantage — something he’s not going to have in the general election.”