Mitt Romney has tightened his grip on the Republican presidential nomination, sweeping three primaries in Wisconsin, Maryland and Washington, DC, and urged his party to shift focus to the general election showdown with President Barack Obama.
The victories enabled Mr Romney to extend 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 with Fox News.
Despite the setbacks, Mr 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 Mr 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 Mr Romney gaining 48% of the vote, compared with 30% for Mr Santorum, 11% for Newt Gingrich, the former speaker of the House of Representatives, and 9% for Texas Representative Ron Paul.
In Wisconsin, with 12% of the vote counted, Mr Romney had 43% to 38% for Mr Santorum, 11% for Mr Paul and 6% for Mr Gingrich.
With 28% of precincts counted in Washington, Mr Romney had 69% of the vote to 13% for Mr Paul and 11% for Mr Gingrich. Mr Santorum was not on the ballot.