Mitt Romney is closer to the Republican presidential nomination by winning primaries in Wisconsin, Maryland and Washington DC.
Mr Romney beat his main rival Rick Santorum in Maryland, although his projected margin of victory looked leaner in Wisconsin.
Mr Santorum, who has faced calls to bow out of the race in the name of party unity, vowed to fight on.
Wednesday's wins put Mr Romney over halfway to getting the number of delegates needed to become the Republican nominee for president.
He has now won 24 out of 37 contests and amassed about 640 delegates.
The eventual winner will face President Barack Obama in November's election.