U.S. television networks project former Massachusetts governor Mitt Romney has swept all three Republican presidential primary elections Tuesday — in Washington DC, the eastern state of Maryland, and the midwestern state of Wisconsin.

Tuesday's victories bring him another step closer to having enough delegates to become the Republican Party's choice to take on President Barack Obama, a Democrat, in the November general election.

Romney already has about half the number of delegates needed for the nomination. He has also been endorsed by such major Republicans as former President George H.W. Bush.

But Romney's Republican challengers — Rick Santorum, Newt Gingrich, and Ron Paul — have given no signs they are ready to drop out of the race.

Santorum, a former U.S. senator from Pennsylvania, is hoping his home state's primary later this month will give him a badly needed boost.

He told supporters in Pennsylvania Tuesday night that the Republican race is only at the halfway mark and says he plans to come out charging into the second half.

Meanwhile, President Obama is already anticipating a November showdown against Romney. In a speech to newspaper executives Tuesday, Mr. Obama denounced a Republican budget plan backed by Romney, saying it would impose “a radical vision” on the country.

Vice President Joe Biden and Secretary of State Hillary Clinton have also criticized Romney, and Democratic “attack ads” targeting Romney also have been appearing on radio and television.