So far, five of seven leading candidates invited by the Family Leader to a "Leadership Summit" on July 12 declined invitations, reports the Des Moines Register.
Those who already said they will not attend include New Jersey Sen. Cory Booker, former Texas Rep. Beto O'Rourke, Massachusetts Sen. Elizabeth Warren, California Sen. Kamala Harris and South Bend, Ind. Mayor Pete Buttigieg.
As of Thursday evening, former Vice President Joe Biden and Vermont Sen. Bernie Sanders had not responded regarding the invitation. Neither has RSVP'd affirmatively.
New Jersey Sen. Cory Booker called out the Family Leader for espousing hateful views.
"I welcome any opportunity to talk about how faith guides me, but I cannot--in good conscience--attend an event put on by an organization that preaches bigotry and sows hate against the LGBTQ community," he wrote on Twitter.
The Iowa-based organization and its CEO, Bob Vander Plaats, says he wants the candidate to attend so that they can understand their positions on religion.
"A lot of them are using faith as kind of a focal point, as a centerpiece," said Vander Plaats just last week, president of the Family Leader. "So we'd like to find out what makes them tick."
Vander Plaats and his group are most well-known for campaign against marriage equality for years. In 2012, the group signed a pledge to defend the "Institution of Marriage as only between one man and one woman" a condition of its endorsement,"
In 2010, the group successfully campaigned to have three state Supreme Court justices booted from the court following a decision supporting marriage equality.
Group leaders said the event was intended as a forum for civil discourse and that Democratic candidates should attend. They also told the paper that Vander Plaats once had a gay friend, lesbian activist Donna Red Wing.