ST. PAUL  (AP) — It's now up to voters to decide whether to ban gay marriage in Minnesota.

A proposed constitutional amendment that would define marriage as between only a man and a woman was approved Saturday night in the Minnesota House. It was the last step needed to put the question on the statewide ballot in November 2012.

State law already prohibits gay marriage in Minnesota, but supporters of the proposed amendment say it's necessary to prevent judges or lawmakers from legalizing it in the future.

Opponents say the constitution should be used to expand rights, not limit them, and predicted a long, divisive debate over the next 18 months.

Democratic Gov. Mark Dayton has no power to block the amendment, but calls it "un-Minnesotan" and is vowing to fight it.

Copyright 2011 The Associated Press.