AP

WASHINGTON

REPUBLICAN front-runner Mitt Romney and President Barack Obama are trading rhetorical punches as though the former Massachusetts governor has already locked up his party’s nomination.

That appeared ever more certain as Romney swept to victory on Tuesday in primary contests in Wisconsin, Maryland and Washington, DC, further diminishing the hopes of conservative challenger Rick Santorum.

While the former Pennsylvania senator vowed to stay in the race, Romney’s wins strengthened the likelihood that he will seal the nomination by the end of the primary season in June, if not before.

And senator John McCain, defeated by Obama in the 2008 election, joined the growing group of Republicans calling for Santorum to end his campaign.

Santorum should recognise “it’s time for a graceful exit,” McCain said on Wednesday. Speaking on CBS television, McCain said there is a strong field of Republicans who could be the vice presidential candidate and he believed it should be Sarah Palin, his running mate four years ago.

Romney planned to speak on Wednesday on the same Washington stage from which Obama a day earlier attacked Romney by name.

That marked the ringing of the opening bell in their fight for the White House that won’t end until the November 6 election.

Romney planned to address the Newspaper Association of America and the American Society of News Editors.

Obama spoke to the annual meeting of The Associated Press.

“There is a basic choice before us,” Romney said on Tuesday night as he spoke to cheering supporters.

“Our different visions for America are the product of our values and our life experiences.” Romney didn’t mention Santorum. He instead sought to cast Obama as an “out of touch” liberal whose personal background is hostile to a free economy.

Obama, hours earlier, had attacked the budget plan supported by Romney and approved last month by House Republicans as “thinly veiled social Darwinism” that “is antithetical to our entire history as a land of opportunity and upward mobility for everybody who’s willing to work for it.” Obama called it “a prescription for decline.” Later on Wednesday, Romney travels to a campaign event in Pennsylvania, hoping to deal a knockout blow to Santorum in his home state in the April 24 primary.

For Romney, the end of the contested primary campaign could hardly come soon enough.

Obama has gained in the polls in recent months, particularly among women, as Republicans fought among themselves for support from the party’s increasingly conservative base.

Also working in Obama’s favour are polls showing Americans are growing more optimistic about the economy.

Unemployment has fallen in recent months, but it is still at a relatively high 8.3 percent.

When he wasn’t focusing on Obama, Romney prodded Santorum to quit the race, suggesting a refusal to do so could cost the party the election.

“I want to have our nominee start raising money, start organising a national campaign and focus on President Obama and his agenda because this is time for us to start focusing on him rather than standing and focusing on one another in these primary contests,” Romney told conservative radio host Sean Hannity on Tuesday.

History explains why the Republicans need to quickly form up behind one candidate.

Just one Republican, Ronald Reagan‚ has defeated a Democratic incumbent president in the last century.

And Romney faces an incumbent with five times more staff, 10 times more money and the world’s greatest bully pulpit.

Santorum took note but dismissed calls for to exit the race.

He vowed to compete in the Pennsylvania primary and in contests next month in Texas, Arkansas, and other southern states. He had three campaign events scheduled across Pennsylvania on Wednesday.

Romney won at least 83 delegates in Tuesday’s three primaries, with six delegates yet to be decided.That pushed his total to 655 of the 1,144 delegates needed to clinch the nomination at the Republican National Convention in August.

Santorum has 278 delegates, Newt Gingrich 135 and Republican Ron Paul 51.

Romney is on pace to reach the number of delegates needed to clinch the nomination in early June.