Washington, Apr 4 (IANS): With decisive victories in Republican presidential primaries in three states, frontrunner Mitt Romney virtually assured himself of the party nomination to challenge President Barack Obama in the November poll.

Romney's wins Tuesday in Wisconsin, Maryland and Washington DC will put him past the halfway mark to the 1,144 delegates needed to clinch the nomination and add to a wide delegate lead that he holds over the other three other candidates still in the race, according to CNN estimates.

"Thank you to Wisconsin, Maryland and Washington. We won them all," Romney told supporters Tuesday night in Milwaukee. "This has really been quite a night. We won a great victory tonight in our campaign to restore the promise of America."

The former Massachusetts governor is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates, with some designated solely for the winner and the rest to be awarded proportionally. He will take all 16 delegates at stake in Washington.

Analysis said the losses in the three states, particularly Wisconsin, may damage closest rival former senator Rick Santorum's ability to convince Republicans that he can stop Romney from getting the nomination.

Pre-primary polls appeared to show Wisconsin's primary was the only one of the three where Santorum had a chance to win.

While Romney is far ahead of Santorum, Former House speaker Newt Gingrich and House member Ron Paul in the hunt for delegates, exit polls indicate Romney still has a problem locking in the conservative base of the party.

That continues to fuel Santorum's campaign, in which he continually depicts himself as the lone true conservative going up against the Republican establishment and liberal media bias.

Obama, meanwhile, clinched the Democratic presidential nomination Tuesday with primary wins in Maryland and Washington, CNN projected. Unlike the Republicans, Obama faces no serious opposition in his race.