Romney wins Wisconsin, Maryland and DC primaries
Mitt Romney has taken a stride closer to the Republican presidential nomination by winning primaries in Wisconsin, Maryland and Washington DC.
He routed his main rival Rick Santorum in Maryland, winning nearly 50% of the vote, and appeared set for a five-point victory in Wisconsin.
Mr Santorum, who has faced calls to bow out of the race in the name of party unity, defiantly vowed to fight on.
The eventual winner will face Barack Obama in November's election.
President Obama attacked Mr Romney earlier on Tuesday in the latest sign that he views the former Massachusetts governor as his November election opponent.
Former House Speaker Newt Gingrich and Texas Congressman Ron Paul trailed far behind in the primary results.
In Washington DC, where Mr Santorum was not on the ballot, Mr Romney took 70% of the vote.
The three-state win meant Mr Romney took home some 83 nominating delegates, substantially increasing his lead over Mr Santorum, whose poor showing gained him just six delegates, the Associated Press projected.
The first to win 1,144 delegates will become the party's presidential nominee. After Tuesday's wins Mr Romney leads Mr Santorum by 655 delegates to 278, AP projected.
Analysts say the former Massachusetts governor is now close to a tipping point where Mr Santorum would be unable to catch him in the race.
Exit polls indicated that Republican voters were primarily concerned with a candidate's ability to defeat Mr Obama.
In his victory speech on Tuesday, Mr Romney focused on the president and not on his Republican opponents, describing Mr Obama as an "out-of-touch liberal" and blaming him for home foreclosures, government debt and his fuel prices.
More Americans had lost their jobs under the Obama administration than under any other president since the Great Depression, Mr Romney said.
"These last few years have been difficult, made worse by mistakes and failures of leadership," he added.
Mr Santorum only seriously competed in Wisconsin, an agricultural and manufacturing state that was the night's biggest prize.
Wisconsin provided his best result of the night giving him 37.6% of the vote compared to Mr Romney's 42.5%, according to returns from 98% of precincts.
In his speech on Tuesday night, the former senator pledged to fight on to the primary in his home state of Pennsylvania on 24 April.
He told cheering supporters: "We have now reached the point where it's half-time - half the delegates in this process have been selected. Who's ready to charge out of the locker room in Pennsylvania for a strong second half?"
Longshot candidate Mr Gingrich, a former House of Representatives Speaker, also vowed on Tuesday to continue in the race - all the way to the Republican convention in August.
Mr Romney and his allies have spent an estimated $53m (£33m) on television advertising so far this election cycle, compared with $27m from his three Republican competitors combined.
Mr Obama took aim at the Republican front-runner on Tuesday in a stinging critique of the budget proposal recently laid out by a high-profile Romney ally, Wisconsin Congressman Paul Ryan.
Addressing news executives in Washington DC, President Obama said the so-called Ryan budget was "thinly veiled social Darwinism".
He condemned Mr Romney for backing the budget, saying: "He even called it marvellous, which is a word you don't often hear when it comes to describing a budget."
Mr Obama'sre-election campaign is also running a TV adin six swing states criticising Mr Romney by name for the first time - in this case as a backer of "Big Oil."