A national group that has spent millions to defend the traditional definition of marriage has pledged to spend $500,000 against any Republicans who vote to legalize same-sex marriage.

The move comes less than a week after Republican state Sen. Branden Petersen of Andover said he was considering becoming a co-author of a proposal to legalize same-sex marriage in Minnesota.

“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 Brian Brown, president of the National Organization for Marriage. “NOM will do everything in our power to defeat any Republican who votes in favor of same-sex marriage.”

Petersen could not immediately be reached for comment. Minnesotans United, the group leading the effort to legalize same-sex marriage, said the National Organization for Marriage has not always followed through with threats to oust legislators who oppose their views.

"This desperate attempt by the National Organization for Marriage to financially bully legislators to vote against their values clearly shows they know that Minnesotans want to secure the freedom to marry for same-sex couples in 2013," said Richard Carlbom, campaign manager for Minnesotans United. "Leaders who help secure the freedom to marry will be rewarded by voters -- just like in every other state."

The National Organization for Marriage noted that they helped "take out” three of four New York GOP Senators who voted to legalize same-sex marriage in that state “by repeatedly informing their constituents of their betrayal on marriage.”

“We will not hesitate to do the same thing in Minnesota,” Brown said.

The head of the Minnesota Republican Party said he is not surprised by the group's determination to persuade legislators, but the party does not plan to punish legislators who vote for same-sex marriage.

Pat Shortridge, chairman of the Minnesota Republican Party, said local activists will decide the fate of GOP legislators, regardless of which position they take on marriage.

"I'm sure the candidate's views on marriage will come up, but so will their views on taxes, spending, regulation, health care, education, life, the 2nd amendment, and a host of other important issues," Shortridge said.