Rep. Benjamin L. Cardin has increased his lead in the contest for the Democratic nomination in Maryland's U.S. Senate race and likely Republican nominee Lt. Gov. Michael S. Steele remains highly competitive in any general election matchup, a poll released today shows. Likely voters in the Sept. 12 Democratic primary now favor Mr. Cardin over ex-Rep. Kweisi Mfume, a former National Association for the Advancement of Colored People president, by 43 percent to 30 percent, according to a statewide poll by nonpartisan Gonzales Research & Marketing Strategies. In the general election, the poll found Mr. Steele trailing Mr. Cardin 44...