The American Federation of State, County and Municipal Employees is now the biggest outside spender of the 2010 elections, thanks to an 11th-hour effort to boost Democrats that has vaulted the public-sector union ahead of the U.S. Chamber of Commerce, the AFL-CIO and a flock of new Republican groups in campaign spending.

The 1.6 million-member AFSCME is spending a total of $87.5 million on the elections after tapping into a $16 million emergency account to help fortify the Democrats' hold on Congress. Last week, AFSCME dug...