WASHINGTON — A new series of political advertisements on behalf of an embattled Nebraska senator could open the door to a flood of similar ads financed by outside groups and even businesses working directly with political candidates — a sharp departure from past practice. The ads are innocuous enough on their face: Senator Ben Nelson, a Nebraska Democrat up for re-election next year, is featured on television and radio commercials discussing Social Security, the national debt, war veterans and other hot-button issues. What is remarkable, campaign finance lawyers and political operatives say, is that the ads were produced and paid...
