About a third of the University of Michigan law school's graduating class walked out on Ohio Senator Rob Portman's commencement speech over his anti-gay record, the A2 Journal reported.

In January, Portman returned to Congress, succeeding retiring Senator George Voinovich, after a six year absence. For 12 years ending in 2005, Portman represented Ohio 2nd congressional district in the House.

The 55-year-old Republican in 2004 voted in favor of a constitutional amendment banning gay marriage. And in 1999 he supported a bill that sought to ban gay and lesbian couples in the District of Columbia from adopting children.

Students protested the decision to invite Portman, a 1984 alum of the law school, to address the class at Saturday's Senior Day.

More than 100 students, or about one third of the class, quietly filed out of the auditorium in protest over Portman's anti-gay votes. Many students also wore symbols of gay pride, including rainbow tassels on their caps. The students returned after Portman had concluded his speech.

School officials previously threatened to remove students who interfered “unduly with a speaker's freedom of expression.”