Mitt Romney has won the Maryland and Washington DC Republican primaries in the US, boosting his momentum as front-runner in the battle for his party's presidential nomination, local TV networks projected.
Shortly after polls closed, the networks declared the former Massachusetts governor the victor over his Republican rivals Rick Santorum, Newt Gingrich and Ron Paul.
According to opinion polls, Romney had been projected to win all three primaries of Wisconsin, Washington DC and Maryland.
If he goes on to win Wisconsin - where polls have closed - it will give him nearly unassailable lead to get his party’s nomination to face President Barack Obama in November.
All eyes are now on Wisconsin, a Midwestern heartland state where Santorum had hoped to break Romney's momentum. Romney came from behind in the last couple of weeks to surge to the top of the polls here.
A sweep of all three of Tuesday's contests would underscore Romney's growing strength and likely increase appeals from party leaders for Republicans to rally behind him, despite deep reservations among many conservatives suspicious about whether he is one of them.
Winning all three contests could give Romney 98 more delegates, putting him well over half of the 1,144 needed to clinch the nomination at the Republican convention in August.
And it would set the tone for the next big date on the campaign calendar, April 24, when six states hold Republican presidential contests.
Romney leads in five of them and plans to make an aggressive push in the sixth, Santorum's home state of Pennsylvania.
A Quinnipiac University poll released on Tuesday showed Santorum ahead of Romney there by 41 to 35 per cent.