Mitt Romney was set to take a big step toward the Republican US presidential nomination yesterday with victories in Wisconsin and two other contests that would increase the pressure on rival Rick Santorum to drop out of the race.

The grinding, months-long battle for the right to face Democratic President Barack Obama in the November 6 election turns to Wisconsin, Maryland and Washington DC, and Romney leads in all three.

A sweep would under-score front-runner 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.

Practically, winning all three contests would give Romney 95 more delegates and put him at well over half of the 1,144 needed to clinch the nomination at the party's 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.