Mitt Romney is increasingly turning his firepower on Barack Obama (AP)
Republican front-runner Mitt Romney and president Barack Obama are trading blows as though the former Massachusetts governor has already locked up his party's nomination.
That appeared ever more certain after Mr 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, Mr 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 John McCain, defeated by Mr Obama in the 2008 election, joined the growing group of Republicans calling for Mr Santorum to end his campaign. He should recognise "it's time for a graceful exit," Mr McCain said.
He added 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.
"There is a basic choice before us," Mr Romney said."Our different visions for America are the product of our values and our life experiences." He did not mention Mr Santorum. He instead sought to cast Mr Obama as an "out of touch" liberal whose personal background is hostile to a free economy.
Mr Obama, hours earlier, had attacked the budget plan supported by Mr 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."
For Mr Romney the end of the contested primary campaign could hardly come soon enough. Mr 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 Mr 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. %.
Mr Romney now has 658 delegates, putting him on pace to reach the 1,144 delegates needed to clinch the nomination by early June. Mr Santorum has 281 delegates, Newt Gingrich 135 and Ron Paul 51.