Republican presidential candidate, former Massachusetts Gov. Mitt Romney shakes hands with a little girl after a campaign in Gilbert, South Carolina, Jan. 20, 2012. (Xinhua file photo/Fang Zhe)

U.S. Republican presidential hopeful Mitt Romney on Tuesday swept primaries in Wisconsin, Maryland and District of Columbia, further solidifying his frontrunner status, putting more pressure on main rival Rick Santorum to quit.

According to projections made by major television networks including CNN, NBC and Fox News, Romney won the trifecta with comfortable margins, and is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates. He is expected to get all of D.C.'s 16 delegates.

With 45 percent of the votes reported in Wisconsin, Romney had 43 percent and Santorum, a former U.S. senator from Pennsylvania, had 38 percent. U.S. Rep. Ron Paul of Texas had 11 percent, and former House Speaker Newt Gingrich had 6 percent.

In Maryland, with 41 percent of the votes reported, Romney had 48 percent. Santorum had 30 percent, Gingrich had 11 percent and Paul had 10 percent.

In the District of Columbia, with 32 percent of the votes reported, Romney had 68 percent. Paul had 13 percent, Gingrich had 11 percent, and Santorum was not on the D.C. ballot.

The results Tuesday added to Romney's lead in delegate count. CNN estimated Romney will have 648 delegates, inching closer to 1144 delegates that would make him the Republican party nominee, while Santorum lags behind with 264 delegates. Gingrich and Paul have 137 and 71 delegates respectively.

The loss in Wisconsin raised serious doubts over Santorum's ability to stop Romney from amassing the 1144 delegates. However, a tired-looking Santorum told supporters in his home state Pennsylvania he's only beginning the "second half" of the campaign, claiming an expected win later this month in Pennsylvania would make the landscape different.

In Milwaukee, Romney targeted his victory speech at President Barack Obama, casting the president as "out of touch." With his big lead in delegates, Romney already acted like the de facto nominee, and rarely mentions his GOP rivals on the campaign trail.

Meanwhile, on the Democratic side, after the races Tuesday, Obama has passed his party's delegate requirement of 2778 delegates to clinch the nomination. He runs unopposed in the party.