A coalition calling itself Freedom Indiana was able to push a planned November vote on the resolution on to a 2016 ballot, the Journal Gazette in Fort Wayne reported Tuesday.
Calling members of the group "underdogs," Freedom Indiana's campaign manager, Megan Robertson, said "our success reflects the strength of the incredible coalition we were able to build in just six months."
"We can finally breathe a collective sigh of relief that lawmakers are finished with the amendment this session, and it will not appear on the ballot this November," she added.
The amendment would define marriage as between one man and one woman. Legislators earlier removed a second sentence that would ban anything "substantially similar," such as civil unions.
The 2015 Legislature must approve the same language before it appears on state ballots in 2016.
Indiana state law already bans same-sex marriage, but the bill's author, Republican state Sen. Dennis Kruse, said the proposed amendment was "good for our state and for our citizens."
Five Republicans voted against the measure, including state Sen. Mike Delph. While he supported the proposal, Delph said its current form did little to protect traditional marriage.
Senate President Pro Tem David Long supported the proposal's final wording, saying it was "clear, unequivocal, does not have baggage."
He said the resolution "could have crashed and burned this fall on the second sentence."