Front-runner Mitt Romney sweeps 3 primaries

Mitt Romney tightened his grip on the Republican presidential nomination Tuesday night, sweeping primaries in Wisconsin, Maryland and Washington, D.C., and shifted his focus to the general election with a sharp attack on 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.    

Despite the setbacks, Santorum, a former Pennsylvania senator, told supporters in his home state that he does not intend to quit the race.     

Wisconsin was the marquee contest of the night, the only place of the three voting Tuesday where Santorum mounted a significant effort. Romney's victory there marked his fourth in little more than a month in a belt of Midwestern industrial states that also included Michigan, Ohio, and Illinois.     

Returns from 95 percent of Wisconsin's precincts showed Romney with 42 percent of the vote to 38 percent for Santorum, 12 percent for Texas Rep. Ron Paul and 6 percent for Newt Gingrich, the former speaker of the House of Representatives.     

Returns from 99 percent of Maryland's precincts showed Romney with 49 percent of the vote to 29 percent for Santorum, 11 percent for Gingrich and 10 percent for Paul.     

With all precincts counted in Washington, Romney had 70 percent of the vote to 12 percent for Paul and 11 percent for Gingrich. Santorum was not on the ballot.     

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.     

AP