Mitt Romney can take a big step toward the Republican U.S. presidential nomination on Tuesday 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 D.C., and Romney leads in all three.

A sweep would underscore 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. He travels there on Wednesday.

Wisconsin is the most closely watched race of the trio voting on Tuesday because Santorum, a conservative former U.S. senator, has campaigned heavily in the state and had led in the polls until ceding the lead to the Romney, the former Massachusetts governor and private equity executive, in the last week or so.

Sensing the nomination is in sight, Romney has made no mention in recent days of Santorum or his other Republican rivals, Newt Gingrich and Ron Paul, preferring instead to pivot to a general election campaign against Obama over the president's handling of the U.S. economy.

While the economy has been showing signs of progress, Romney says high unemployment, high poverty and burdensome regulations remain serious challenges that prove Obama has been a failure.

'Strategy was a bust'

"His economic strategy was a bust," Romney told supporters in Milwaukee on Monday. "And one of the reasons why we're going to take over the White House is because he doesn't know how this economy works, and we do."

If he does win the nomination as expected, Romney would face the challenge of defeating an incumbent president whose campaign operation is well-funded, organized and eager to pounce on any misstep. Romney also would enter the general election campaign at a distinct disadvantage among women voters, who could be critical in swinging the election.

Santorum wants to survive Wisconsin and the rest of April and move on to May, where the states that vote may be more favorable to him. He would have to win an overwhelming percentage of the remaining delegates to win the nomination outright.

But Santorum seems to have a different strategy: Win enough delegates to deny outright victory to Romney. This would force Republicans to choose their candidate at a "brokered" convention in Tampa, a chaotic scenario that many political experts believe could be disastrous to the party's hopes of ousting Obama.

"I would argue even if it ends up in a convention, that's a positive thing for the Republican Party, that's a positive thing for activating and energizing our folks heading into this fall election," he told reporters on Monday.

Trying to appeal to blue-collar voters, Santorum has held small campaign events in Wisconsin, frequently appearing in bowling alleys. He has been relentless in his attacks on Romney as a Massachusetts moderate who would govern little differently than Obama. He insists he is staying in the race.

Romney has benefited from the endorsement in Wisconsin of Paul Ryan, the popular conservative from the state who is chairman of the powerful House of Representatives Budget Committee.

The two men have appeared side by side at multiple campaign events and appear to have a warm relationship, leading to news media speculation that Ryan could ultimately be on Romney's short list for the vice presidential nomination.