The Indiana Senate on Tuesday overwhelmingly agreed to amend the state's constitution to ban gay marriage, the AP reported.

Three Democrats crossed the aisle to join all 37 Republicans in a 40 to 10 vote.

The proposed amendment would define marriage as a heterosexual union, and outlaw gay marriage, civil unions and possibly domestic partnerships.

The Republican-controlled House approved the measure with a lopsided 70-26 vote late last month.

Before going to the ballot box, however, a second separately elected General Assembly must approve the resolution, making 2014 the earliest voters could act.

The measure was sponsored in the Senate by Republican Senator Dennis Kruse, who said the amendment was needed to affirm “that marriage is and should be the union of one man and one woman.”

During a House hearing last month, Micah Clark of the Christian conservative American Family Association of Indiana testified that the state's law banning gay marriage remained vulnerable to a legal challenge.

“Two-thirds of the states have amended their constitutions because of the legal challenges that have occurred,” Clark said. “For example, Iowa had a law similar to ours [and then] had same-sex marriages forced on them by the courts.”

However, Indiana courts have already declared the state's law constitutional.