US election 2012: Mitt Romney looks to pull clear in three Tuesday primaries

Mitt Romney looked to pull further away from his rivals for the Republican nomination in three primaries on Tuesday as the campaign focus shifted to the November contest against US President Barack Obama.

Mitt Romney currently holds a 7.5-point lead in the polls

Primary polls opened in Maryland and the US capital Washington at 7:00am (1100 GMT), and opened an hour later in Wisconsin, the state expected to be the most closely fought contest.

The party establishment is coalescing behind the former Massachusetts governor, with top Senate Republican Mitch McConnell saying Mr Romney is all but certain to be the party's nominee to run against Obama in November.

Wins on Tuesday in the three contests would likely push Romney past the halfway mark in terms of the number of delegates he needs to clinch the presidential nomination at the party convention in August.

In the tightest race, in Wisconsin, where Romney and main rival Rick Santorum campaigned on Monday, Mr Romney holds a 7.5-point lead in polls, according to the RealClearPolitics website.

Mr Romney spent much of Monday chastising the Democratic incumbent and his "government-centred society" for failing to lead a recovery out of a deep recession and "crushing" the American dream.

But Mr Obama is also easing into campaign mode, and planned to deliver a speech on Tuesday accusing congressional Republicans of championing a cut-throat "social Darwinism" that neglects the middle class and favours the wealthy.

While most signs point to Mr Romney as the party's eventual flag-bearer, his prospects get cloudier looking beyond the pitched battle for the nomination, with a new USA Today/Gallup poll released Monday showing him trailing Mr Obama.

The poll has Mr Obama leading 49 to 45 per cent over Romney nationally among registered voters in a hypothetical one-on-one matchup, the largest lead to date in Gallup polling.

Mr Obama's lead in 12 swing states was even greater, with the president ahead 51-42 on average in key states like Florida, Ohio and Pennsylvania.

That marks a dramatic shift in fortunes for Mr Obama, who a month ago trailed Romney by two percentage points in swing states.

The numbers also showed waning enthusiasm among Republican voters.

"The decline is especially apparent among Romney voters, whose enthusiasm has fallen 13 percentage points from January, and now is on par with Obama voters' enthusiasm," Gallup researchers reported.

Mr Romney has faced stubborn scepticism by conservative voters, who fear that the former governor of liberal Massachusetts will tack to the left once he wins the nomination in order to appeal to independents.

That scenario is fodder for Santorum, a harsh critic of abortion and gay rights who has tapped into conservative angst about the front-runner.

But McConnell said Mr Romney "is going to be an excellent candidate, and I think the chances are overwhelming that he will be our nominee".

"It is in the best interest of our party to get behind (Romney) and make the case against the president of the United States."

Santorum, a former Pennsylvania senator, insisted the race was "a lot closer than Mitt Romney and the pundits are spinning" in an interview with CNN, referring to a string of more conservative state primaries scheduled for May.

Romney has won 21 out of 34 contests so far and amassed some 565 delegates of the 1,144 needed for the nomination. Santorum has racked up 11 victories but has less than half Romney's delegate count.