MILWAUKEE - Mitt Romney won contests in Maryland and Washington DC on Tuesday to tighten his grip on the race for the Republican presidential nomination and looked to land a big blow on rival Rick Santorum with an expected victory in Wisconsin.

It was a strong start for the front-runner on a big day in the Republican search for a candidate to send against Democratic President Barack Obama in the Nov 6 election.

Television networks projected that the former Massachusetts Governor Romney was the easy winner over Santorum, Newt Gingrich and Ron Paul in Maryland. NBC predicted Romney also won in the nation's capital.

US Republican presidential candidate Mitt Romney greets supporters after speaking at a pancake breakfast in Wauwatosa, Wisconsin, April 1, 2012.  [Photo/Agencies]

All eyes were on Wisconsin, a Midwestern heartland state where Santorum had hoped to break Romney's momentum.

In a sign that Obama sees Romney as his chief obstacle to re-election, the president singled him out by name on Tuesday for backing a controversial Republican budget plan. Obama's sharp partisan tone, and Romney's swift and withering response, foreshadowed the campaign battles to come.

"He said that he's very supportive of this new budget and he even called it 'marvelous,' which is a word you don't often hear when it comes to describing a budget," Obama said in a speech.

Romney fired back that Obama was trying to deflect blame for high gasoline prices that are hitting American wallets hard and could make Obama's re-election tougher to achieve.

"So the president put an ad out yesterday, talking about gasoline prices and how high they are. And guess who he blamed? Me!" Romney said in Waukesha, Wisconsin.

"Maybe after I'm president I can take responsibility for things I might have done wrong. But this president doesn't want to take responsibility for his mistakes."

Wisconsin battle

Romney came from behind in the last couple of weeks to surge to the top of the polls in Wisconsin, and a loss would further diminish the conservative former US senator Santorum's hopes of a miracle finish to wrestle the nomination from his more moderate opponent.

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 percent.