Rep. Benjamin L. Cardin yesterday tried to rally support among black Democrats in Prince George's County, while Lt. Gov. Michael S. Steele campaigned with county Democratic leaders who broke ranks this week to endorse him. Mr. Cardin appeared with Prince George's County Executive Jack B. Johnson and about 30 other lawmakers in Hyattsville, aiming to keep a diminishing lead in the polls. In his first public show of support for Mr. Cardin, Mr. Johnson ... But the rally turned sour later. State Sen. Nathaniel Exum, Prince George's Democrat, berated Mr. Cardin for excluding him and other local delegates from speaking...