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Mitt Romney has swept a trio of GOP primaries, coasting to wins over chief rival Rick Santorum in Wisconsin, Maryland and Washington, D.C.

The wins propel him closer to winning the GOP nomination, which he is expected to clinch by June. Santorum declared that only half the country has cast votes in the contest and vowed to stay in the race. Newt Gingrich and Ron Paul also remain in the race. Neither is a threat to Romney.