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MILWAUKEE - Mitt Romney tightened his grip on the Republican presidential nomination Tuesday night, sweeping three primaries in Wisconsin, Maryland and Washington, D.C., and urged his party to shift focus to the general election showdown with President Barack Obama.

The victories enabled Romney to pad his already considerable delegate lead over chief Republican rival Rick Santorum, who flashed defiance in the face of growing pressure to abandon his own candidacy in the name of party unity.

"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," the former Massachusetts governor said in an interview Tuesday with Fox News.

Despite the setbacks, Santorum, a former Pennsylvania senator, told supporters in his home state that he does not intend to quit the race. He vowed to keep battling until Romney is assured of the 1,144 delegates needed to secure the nomination at the party's national convention in late August in Tampa, Florida.

Maryland returns showed Romney gaining 48 per cent of the vote, compared with 30 per cent for Santorum, 11 per cent for Newt Gingrich, the former speaker of the House of Representatives, and 9 per cent for Texas Rep. Ron Paul.

In Wisconsin, with 12 per cent of the vote counted, Romney had 43 per cent to 38 per cent for Santorum, 11 per cent for Paul and 6 per cent for Gingrich.

With 28 per cent of precincts counted in Washington, Romney had 69 per cent of the vote to 13 per cent for Paul and 11 per cent for Gingrich. Santorum was not on the ballot.

Romney and Obama exchanged barbs Tuesday in further acknowledgement that both camps think the fall campaign for the general election has already begun.

"Four more years?" Romney asked sarcastically of the president as supporters cheered him in Milwaukee.

He said Obama was "a little out of touch" after spending four years surrounded by the trappings of power and had presided over near-record job losses as well as increases in poverty, home foreclosures, government debt and gasoline prices.

Romney, who has cemented his lead over Santorum through overwhelming spending on television advertising, will face a better organized, better financed Obama campaign backed by the power of the presidency.

"He gets full credit or blame for what's happened in this economy, and what's happened to gasoline prices under his watch, and what's happened to our schools, and what's happened to our military forces," Romney told supporters gathered at a sandwich shop in Waukesha, Wisconsin. "All these things are his responsibility while he's president."

Obama said things could be worse -- and predicted they would be if Romney and Republicans got their way. He criticized Romney for supporting the budget plan introduced by Rep. Paul Ryan of Wisconsin, the influential chairman of the House Budget Committee.

In a speech to the annual meeting of The Associated Press, Obama said the House-passed budget supported by Republicans was "antithetical to our entire history as a land of opportunity and upward mobility for everybody who's willing to work for it ... It is a prescription for decline."

Obama's re-election campaign is running a new TV ad in six swing states criticizing the former Romney by name for the first time - in this case as a backer of "Big Oil." Rising gasoline prices are shaping up to be a major campaign issue and the Republicans are trying to blame that on Obama.

Romney responded Tuesday by accusing the president of trying to shift blame for the bad economy.

"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," Romney said after handing out ham, turkey and Italian sandwiches to supporters.

For Romney, the end of the contested primary campaign could hardly come soon enough. Obama has gained in the polls in recent months, particularly among women, as Republicans vie among themselves for support from the party's increasingly conservative base. Santorum has devoted more time to social issues -- including birth control -- than Romney, who has generally stayed focused on economic issues.

Additionally, surveys indicate Americans are growing more optimistic about the overall state of the economy. Unemployment has fallen in recent months, but it is still at a relatively high 8.3 per cent of the work force.

There were 95 Republican National Convention delegates at stake for the day, including 42 in Wisconsin, the only one of the three contests that a fading Santorum seriously contested.

Romney won at least 44 delegates in Maryland and Washington.

That gave him 616 of the 1,144 needed to clinch the nomination and on a pace to do so before the end of the primary season in June. Santorum had 272 delegates, Gingrich 135 and Paul 51.

Interviews with voters leaving Republican polling places in Maryland and Wisconsin showed an electorate more concerned with a candidate's ability to defeat Obama than with the strength of his conservatism, his moral character or his stand on the issues. Similar soundings in earlier states have consistently worked to Romney's advantage.

Voters in both states were less apt to be born again or evangelical Christians than in most previous contests -- 34 per cent in Wisconsin and 32 per cent in Maryland. Based on earlier contests, that, too, suggested an advantage for Romney. Santorum, a staunch social conservative, has done well with evangelical Christians because of his strong opposition to abortion and gay rights.

Santorum, a former Pennsylvania senator, made little or no effort in Maryland, was not on the ballot in Washington, D.C., and concentrated much of his time in Wisconsin in rural areas.

Santorum spent the day in Texas at private fundraisers for his campaign before heading to his home state of Pennsylvania for an election night party near Pittsburgh.

Wisconsin was the fourth industrial state to vote in a little more than a month after Michigan, Ohio and Illinois, a string that Romney has exploited to gain momentum as well as a growing delegate lead in the campaign for the Republican nomination.

The former Massachusetts governor won a close Michigan primary on Feb. 28, then an even closer one in Ohio a week later, followed by a convincing victory in Illinois on March 20. At each turn, he was backed by his own robust, well-financed organization as well as a deep-pocketed super political action committee that assured him of an overwhelming advantage in television advertising.

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.

"With almost unlimited resources, Gov. Romney has not proven to be very effective," Santorum said. "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."