Republican presidential candidate Donald Trump speaks at rally in Muscatine, Iowa. (Daniel Acker/Bloomberg News)

As an influential evangelical Iowan, Bob Vander Plaats experienced what it was like to be friends with Donald Trump. The New York billionaire invited Vander Plaats and his family to visit New York several times, refusing to allow them to pay to stay at one of his hotels and lavishing kindness.

Then Vander Plaats — president of the Family Leader, which is opposed to abortion and gay marriage — decided to endorse Sen. Ted Cruz (R-Tex.) for president instead of Trump. On Tuesday, Trump skewered Vander Plaats on Twitter, calling him "phony" and "a bad guy." The attacks came as Trump announced that Liberty University President Jerry Falwell Jr. had endorsed him.

"It's Donald Trump exposed: He wants to be your friend when he thinks he can get something in return for that friendship," Vander Plaats said Tuesday in an interview. "In Iowa, we look at friendship as what we can do for our friends, not what our friends can do for us."

Vander Plaats said he has seen others receive the same sort of 180-degree treatment from Trump. Trump once praised Fox News's Megyn Kelly but is now trying to have her removed as a moderator for the upcoming GOP debate because he believes she asks unfair questions. The entire Republican establishment was once slammed by Trump but now appears to be in his good graces.

"It just shows a guy that gets tossed about to and fro too easily," Vander Plaats said. "I think that kind of pride and arrogance leads to unstable temperament and judgment."

Vander Plaats said he endorsed Cruz over Trump because the billionaire businessman's record on abortion and gay marriage is "a mixed bag." He said that he was concerned when Trump said at a forum with evangelicals in Iowa last summer that he does not seek forgiveness from God. Vander Plaats was also put off by Trump's attacks on Sen. John McCain (R-Ariz.), a decorated Vietnam War veteran. The final straw: when Trump appeared to mock the physical disability of a New York Times reporter. Vander Plaats said he was also dismayed to hear Trump joke that he could shoot someone and still not lose support.