Mitt Romney has won the Wisconsin primary. And that means he's gone three for three over chief GOP rival Rick Santorum in Tuesday's contests.

Romney also won primaries in Washington, D.C., where Santorum was not on the ballot, and Maryland.

The former Massachusetts governor is on pace to clinch the party's nomination in June if not before.

The Republican presidential hopeful was ready Tuesday to focus on the man he hopes to face in November - President Barack Obama.

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

Romney says Obama has spent the last few years "surrounded by an adoring staff of true believers" and is out of touch with America.

The victories enabled Romney to pad his already-wide delegate lead over Santorum, who flashed defiance in the face of pressure to abandon his own candidacy in the name of party unity.

Wisconsin was the marquee contest of the night, the only place of the three on the ballot where Santorum mounted a significant effort.

"We won `em all," Romney declared, a former Massachusetts governor now the nominee-in-waiting for a party eager to reclaim the White House.

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 a conservative party electorate. Santorum has devoted more time to social concerns - 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 percent of the work force.

Romney won at least 74 delegates in the three races, with 21 yet to be allocated.

That pushed his total to 646 of the 1,144 needed to clinch the nomination. Santorum has 272 delegates, Gingrich 135 and Paul 51.

Increasingly, Romney and many senior figures in his party have begun behaving as if the primaries are an afterthought, hoping to pivot to the fall campaign and criticism of Obama.

"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 said of the president while campaigning in Waukesha, Wis.

Obama, in a speech to the annual meeting of The Associated Press, said a House-passed budget written 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."

When he wasn't focusing his rhetoric on Obama, Romney prodded Santorum to quit the race, suggesting a refusal to do so could cost the party the election in November.

"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 said in an interview with Fox News. "You have to remember that it was Ross Perot that allowed Bill Clinton to win" in 1992, he added, a reference to the Texan who ran as an independent that year.

Santorum, in his home state of Pennsylvania, took note of the calls for him to exit the race.

"Ladies and gentleman, Pennsylvania and half the other people in this country have yet to be heard, and we're going to go out and campaign here and across this nation to make sure that their voices are heard in the next few months."

Santorum 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.

He all but conceded defeat in advance in Wisconsin, retreating to Mars, Pa., for an election night appearance in his home state.

Republican Newt Gingrich said he isn't leaving the party's presidential race and vowed to continue all the way to the nominating convention.

Gingrich said Tuesday he is committed to carrying the banner of bold conservative colors all the way to Tampa to ensure, in his words, "the Republican Party never abandons the timeless conservative principles."

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 presidential nomination. He and a super PAC supporting him have greatly outspent his rivals in state after state.