A national group is vowing to spend $500,000 to defeat any Republican who supports gay marriage in Minnesota, starting with state Sen. Branden Petersen of Andover.

Petersen last week said he was considering co-authoring legislation this session that would legalize gay marriage in Minnesota. On Monday, the National Organization for Marriage pledged big money to defeat Petersen.

"Republicans like Branden Petersen don't realize that not only is voting to redefine marriage a terrible policy, it is also a career-ending vote for a Republican," said the group's president Brian Brown in a press release.

The six-year-old Washington-based group also offered to support Minnesota Democrats who reject legalizing same-sex marriage.

"We urge Democrats in the Minnesota Legislature to vote their values, and not what their party bosses tell them," Brown said. "Standing for true marriage is the right thing to do for Minnesota families, and especially for children. The fact is that Minnesota children, and all children, have a right to expect laws that promote them being raised by a mother and father."

Petersen could not immediately be reached Monday, the Star Tribune reported.

The NOM group says that it helped "take out" three of four Republicans in New York's Legislature after they publicly backed gay marriage.

Richard Carlbom, campaign manager for Minnesotans United for All Families, tells the Star Tribune that the group's announcement was a "desperate attempt" to bully lawmakers. "Leaders who help secure the freedom to marry will be rewarded by voters – just like in every other state," Carlbom said.