Mitt Romney's slow but steady march to the Republican presidential nomination is getting harder to stop. This as he comes out the winner in Wisconsin, Maryland, and the District of Columbia Tuesday night, trading attacks all the way with President Obama. Josh Robin has the recap.

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UNITED STATES -- Mitt Romney won Tuesday's Republican presidential primaries in Wisconsin, Maryland and the District of Columbia, placing him closer to the GOP nomination in terms of delegates.

Wisconsin, which has 42 delegates at stake, is a midwestern swing state where polls were closer between Romney and his chief contender, former Pennsylvania Senator Rick Santorum.

Before Tuesday's sweep, Romney had an estimated 572 delegates, or about half of the delegates needed for the nomination.

Santorum had about 272 delegates, former House Speaker Newt Gingrich had 135 delegates and Ron Paul had 51.

The former senator spent the day in his home state of Pennsylvania, which is holding its primary in three weeks, along with New York and three other states.

Meanwhile, President Barack Obama aired a new commercial attacking Romney as beholden to big oil. It is airing in other swing states.