DES MOINES, Iowa - Four of the GOP 2012 hopefuls will take part in a pro-life forum in Des Moines Wednesday night, teaming up with the winner of the 2008 Iowa Republican caucuses, former Arkansas Governor Mike Huckabee.

Candidates Newt Gingrich, Rep. Michele Bachmann, R-Minn., Rick Santorum and Texas Governor Rick Perry will be taking part in the event, where a host of issues about life will be addressed.

Contenders Ron Paul and Mitt Romney will not be in attendance as Paul is campaigning in New Hampshire and Romney is fundraising in New York.

It's just 20 days to the Iowa caucuses and abortion is an important issue for Republicans, particularly evangelicals, who have an influential voice in the party.

Abortion is the stickiest, politically speaking, for Romney, who was pro-choice as governor of the liberal state of Massachusetts and is now pro-life. It's often a charge that his rivals, and Democrats alike, use as an example that Romney is a flip-flopper.

About skipping the forum in Iowa, Romney said , "I am going to be doing events in this area. So, we had a different schedule than the schedule in Iowa."

"My wife is in Iowa today campaigning there. You know there are a lot of states we are campaigning in and we just couldn't-weren't able to be there at that time," he added in an interview with Fox News' Martha MacCallum Wednesday.

Santorum, Gingrich, Bachmann are hard-line pro-lifers. Ron Paul is also pro-life, but as the Libertarian of the bunch, he believes it should be up to the states.

Huckabee, who is also a Fox News host, is hosting the forum at the Hoyt Sherman Place in downtown Des Moines. It's sponsored by the group Citizens United and will be showing the new documentary, "Gift of Life."

The candidates at Wednesday's forum are expected to be asked to sign a pledge stating that a person is defined at the moment of conception and is protected by law, Fox News Correspondent Steve Brown reports.

At least three of the candidates are expected to sign the pledge. Citizens United is the same group that was part of a Supreme Court decision in 2010 that changed campaign finance rules.

On its website the group says the documentary that is showing at the forum "will leave the viewer impressed and inspired by the everyday people who embody the pro-life movement in America. The Gift of Life looks at the lives of individuals who put a face to the pro-life issue, including those who were nearly victims of abortion."