Iowa social conservatives on Tuesday will descend on the Capitol to pressure Senate President Mike Gronstal to allow an amendment banning gay marriage to come up for a vote in the chamber.

Opponents of the Iowa Supreme Court's unanimous 2009 ruling legalizing gay marriage want lawmakers to approve an amendment to the Iowa Constitution which would define marriage as a heterosexual union.

Last year, the Republican-controlled House easily approved the amendment, but Gronstal, a Democrat, blocked the measure from reaching the Senate floor, angering conservatives who have threatened to oust him.

“For two years, Senate President Mike Gronstal has blocked every attempt to bring the marriage amendment to a vote in the Senate,” said Brian Brown, president of the National Organization for Marriage (NOM), a backer of Tuesday's rally. “The amendment already passed the House last year, the first of four legislative votes required before going to the ballot to be ratified by voters.”

Gronstal has previously said that he believes rights should not be put to a popular vote.

“People's rights should not be put to a popular vote,” Gronstal said. “We didn't put slavery to a vote of the people in Iowa. We didn't put the right to go to a school in your neighborhood to a vote of the people, and we didn't put the public accommodations law to a vote of the people in Iowa.”

The Let Us Vote! (LUV) rally in Des Moines is sponsored by the Christian conservative group The Family Leader, which is led by Bob Vander Plaats, a Sioux City businessman whose unsuccessful 2010 campaign to govern the state was fueled mostly on opposition to gay marriage.

“Yes we want to put pressure on Senator Gronstal. It has stalled in the Senate. It's passed in the Iowa House. We want to put pressure on Senator Gronstal to say, 'Let it come up for a vote. Let the people of Iowa vote,'” Vander Plaats said in a video promoting the rally. “And we need you to tell your senator to tell Senator Gronstal to let it come up for a vote. Don't let your senator be the one that gets in the way from letting us vote on marriage.” (The video is embedded in the right panel of this page. Visit our video library for more videos.)

Also involved in the rally is

If lawmakers do not act this year, the earliest voters could see the measure on the ballot would be 2016.