Mitt Romney swept Tuesday's Republican primaries in Wisconsin, Maryland and the District of Columbia, cementing his status of front-runner for the party's presidential nod and making it harder for Rick Santorum to justify staying in the race.

The result in Wisconsin was closer than expected, with Romney winning 42.5 percent of the vote to Santorum's 37.6 percent. U.S. Rep. Ron Paul of Texas finished third with 11.7 percent, while former House of Representatives speaker Newt Gingrich came in last with 6.1 percent.

In Maryland, Romney's victory margin was far more comfortable -- 49.1 percent compared with 28.9 percent for Santorum, the former U.S. senator from neighboring Pennsylvania. Gingrich and Paul got 10.9 percent and 9.5 percent, respectively. 

President Barack Obama also quietly (and unsurprisingly) clinched his party's nomination that night. According to CNN's estimated delegate count, he earned 2,735 of the 2,778 needed to secure his spot.

Romney's rout in the Washington, D.C., primary -- more than 70.2 of voters backed him -- enabled news outlets to call the winner even before official results streamed in. Santorum didn't file to run in the nation's capital leaving the rest of the ballot to Paul, who finished with 12 percent, and Gingrich (10.7 percent). 

But Romney, whose triple victory had been projected in polls before Tuesday's voting, showed again that his focus is squarely on the man he's all but guaranteed to face in November: President Barack Obama.

Addressing supporters in Wisconsin's biggest city, Milwaukee, Romney said the president has become a little out of touch after years of flying around on Air Force One, surrounded by an adoring staff of true believers telling you that you're great and you're doing a great job.

The Republican added: You know, out-of-touch liberals like Barack Obama say they want a strong economy, but in everything they do, they show they don't like business very much.

Romney's attack came hours after Obama, at a gathering of Associated Press editors Tuesday in Washington, blasted the Republicans' latest budget plan as harmful to low-income and middle-class Americans.

It's a Trojan horse. Disguised as deficit reduction plan, it's really an attempt to impose a radical vision on our country. It's nothing but thinly veiled social Darwinism, the president said of the 2013 proposal crafted by Rep. Paul Ryan, the Wisconsin Republican who is chairman of the House Budget Committee. Ryan recently announced his endorsement of Romney to be Obama's Republican challenger this fall. 

Romney is scheduled to address the same AP editors conference Wednesday.

With Tuesday's victories, the former Massachusetts governor adds at least 93 delegates to his total, adding to what the AP estimates is 572 so far. (A candidate will need 1,144 delegates to win the Republican nomination at the party's convention in August.) Santorum is estimated to have 272 delegates, though he claims the tally is higher, due to delegates from nonbinding caucuses in some states. Gingrich has 135 and Paul has 51.

Santorum Sets Sights On Home Turf

Santorum is hoping -- still -- to position himself as the underdog with a chance to win, despite increasing calls for him to quit. It's no surprise he spent the night in his home state, where he looked forward to its Republican primary on April 24.

Ladies and gentlemen, Pennsylvania has yet to be heard, he said to a crowd of supporters in Mars, Pa., a small borough in the western part of the state.

Tonight is the kickoff for the Pennsylvania campaign, Santorum spokesman Hogan Gidley said. Pennsylvania is pivotal for our campaign, but it's also pivotal for Romney. This nomination will be won in the primary and in the general in Pennsylvania.  

Santorum didn't mention his rival by name, but he indirectly accused Romney of being too moderate when he asked Pennsylvanians to pick someone whose views are forged in steel, not on an Etch A Sketch.

We don't win by moving to the middle, Santorum said. We win by getting people in the middle to move to us and move this country forward.

An Inevitable Nominee?

Romney's win comes after a string of endorsements by Republican bigwigs who are encouraging the party faithful to get behind the front-runner and focus on defeating Obama. Increasingly, there are also calls for the other candidates -- including Santorum -- to drop out of the race.

Romney had Ryan, the Wisconsin congressman, by his side while stumping in the Midwestern state this week. Ryan called for his fellow Republicans to unite around Romney because the campaign is getting to the point where it's going to become counterproductive if it drags on, the Milwaukee Journal Sentinel reported.

U.S. Sen. Marco Rubio of Florida chided Romney's competitors for banking on the strategy of a floor fight in Tampa in August because that's a recipe of four more years of Barack Obama. Former Florida governor Jeb Bush has also endorsed Romney.

In another sign the party's establishment is pushing for a Romney nomination, the Republican National Committee plans to begin raising money with Romney this week, the Wall Street Journal reported.

Voters on Tuesday seemed to get the message. According to preliminary exit polls, 8 of 10 voters said they believe Romney will be the nominee, ABC News reported, despite the closeness of the primary. Exit polls showed Romney's recent endorsements were an important factor for about a third of voters in Maryland and Wisconsin, according to CBS News.