The free charter flight for Mitt Romney campaign volunteers seemed like an open-and-shut case for the six members of the Federal Election Commission. A wealthy friend of Romney spent $150,000 to fly as many as 200 campaign volunteers from Utah to a fund-raising phone-a-thon in Boston. The three Democrats on the FEC agreed with the agency’s staff that the charter appeared to violate rules limiting such “in-kind’’ gifts to $2,600 per election. But the three Republican commissioners disagreed, saying Romney’s friend merely acted “in behalf of’’ Romney’s 2008 campaign — not the illegal “on behalf of” — and thus the flight was allowed. With that twist of legal semantics, the case died — effectively dismissed. The 3-3 deadlock was part of a pattern of paralysis that has over the past five years gripped the commission, the nation’s principal referee for federal elections. The FEC has often been the subject of criticism since its founding four decades ago. But the impression of weakness has escalated dramatically, as Republicans named to the panel in 2008, united in the belief that the commission had been guilty of regulatory overreach, have moved to soften enforcement, block new rules, and limit oversight. In essence, according to critics, the FEC has been rendered toothless, and at the worst possible time, when powerful special interests are freer than they have been in decades to exert financial influence on Washington politicians.