Minnesota is poised for a historic day Monday as the Senate prepares to debate a gay marriage bill.

The House debated the measure on Thursday for roughly 3 hours before approving it with 75 votes, including four votes from previously undeclared Republicans. The GOP support surprised some, including House Speaker Paul Thissen.

(Related: MN House Speaker Paul Thissen: GOP support for gay marriage “hugely significant.”)

The issue has molded Minnesota politics like no other in recent times, taking it from blue to red and back again in 2 years.

In 2010, marriage equality foes campaigned heavily in the state as Democrats increasingly came out in support of the issue.

That year, conservative Republican majorities regained control of the House and Senate and immediately began pushing for a referendum on a constitutional amendment which sought to define marriage as a heterosexual union.

Amendment supporters balked when the GOP rejected broader language aimed at banning recognition of any union other than marriage, which would have closed the door to civil unions or domestic partnerships for gay and lesbian couples.

Minnesota voters, in a first, rejected the amendment last fall and returned the House and Senate to Democrats.

The stunning rebuke set the stage for a new push to make Minnesota a marriage equality state.

Supporters say they have the votes in the Senate to make Minnesota the first Midwestern state to legalize such unions without judicial intervention.

If the law is approved, it will take effect on August 1, the same day that gay and lesbian couples in Rhode Island can begin marrying.