Mitt Romney has taken a stride closer to the Republican presidential nomination by winning primaries in Wisconsin, Maryland and Washington DC.He routed his main rival Rick Santorum in Maryland, although his projected margin of victory looked leaner in Wisconsin.Mr Santorum, who has faced calls to bow out of the race in the name of party unity, defiantly vowed to fight on.The eventual winner will face Barack Obama in November's election. President Obama attacked Mr Romney earlier on Tuesday in the latest sign that he views the former Massachusetts governor as his November election opponent. Former House Speaker Newt Gingrich and Texas Congressman Ron Paul trailed far behind in the primary results. In Washington DC, where Mr Santorum was not on the ballot, Mr Romney took 70% of the vote. Exit polls indicated that Republican voters were primarily concerned with a candidate's ability to defeat Mr Obama. In his victory speech on Tuesday, ...