A pair of conservative groups are challenging a new Maine law that limits donations to political action committees that spend independently in candidate elections. The federal complaint filed on Friday was expected. Supporters of the recently approved referendum fully expected a legal showdown over caps on individual contributions to so-called super PACs. The lawsuit by two conservative groups contends the state law that limits individual donations to super PACs at $5,000 runs afoul of the Supreme Court’s Citizens United decision. That 2010 decision opened the floodgates for big business and labor unions to spend freely on elections. Maine’s attorney general and the state’s campaign spending watchdog are named in the suit.