Mitt Romney continued to show why he is the man to beat in the Republican presidential nomination Tuesday, sweeping primaries in Wisconsin, Maryland and Washington D.C.

Returns from 80 percent of Wisconsin's precincts showed Romney with 42 percent of the vote to 38 percent for Santorum, 12 percent for Ron Paul and 6 percent for Newt Gingrich, the Star Tribune said.

Returns from 92 percent of Maryland's precincts showed Romney with 47 percent of the vote to 29 percent for Santorum, 11 percent for Gingrich and 10 percent for Ron Paul.

"We won 'em all," Romney declared, a former Massachusetts governor now the nominee-in-waiting for a party eager to reclaim the White House.

The victories enabled Romney to pad his already-wide delegate lead over Republican rival Rick Santorum, who flashed defiance in the face of pressure to abandon his own candidacy in the name of party unity, the Tribune said.

Romney won at least 74 delegates in the three races, with 21 yet to be allocated.  He now has 646 of the 1,144 needed to clinch the nomination.  Santorum has 272 delegates, Gingrich 135 and Paul 51.

Wisconsin was the only contest of the three where Santorum mounted a significant effort, the Tribune said.  Romney's victory there marked his fourth in little more than a month in the belt of industrial states that also included Michigan, Ohio and Illinois.

Romney has begun behaving as if the primaries were an afterthought, the article said, focusing more on the fall campaign and President Barack Obama.

"He gets full credit or blame for what's happened in this economy and what's happened to gasoline prices under his watch and what's happened to our schools and what's happened to our military forces," Romney said to the president while campaigning in Waukesha, Wis.

When he wasn't focusing on his rhetoric on Obama, Romney prodded Santorum to quit the race, suggesting a refusual to do so could cost the party the election in Novemeber.