Just when Democrats thought they had at least one sure election-year issue, Senate Republicans summoned their 1994 weapon of choice -- the filibuster threat -- to shoot down a campaign-finance-reform bill. "The system stinks," majority leader George Mitchell complained after a vote to close debate on the measure failed 52-46, far short of the 60 needed to forestall a G.O.P. walkout. The bill would have put a new $6,000-per-election-cycle cap on money political-action committees could funnel to a congressional candidate. But Republicans argued that another provision, federal matching money for challengers, amounted to a welfare program for politicians.