When Amy Coney Barnett, a law professor from Notre Dame, began her confirmation hearing to determine whether she should serve on the 7th Circuit Court of Appeals, she was prepared for all the standard-issue questions about things like precedent and jurisprudence. But she probably didn't expect a liberal inquisition. Nobody ever does.

But together, Sens. Dianne Feinstein of California and Dick Durbin of Illinois forced a litmus test on the candidate asking her about "dogma" and probing about "orthodox Catholicism." Unwittingly, they also forced that issue on eight Catholic Democrats running for re-election in 2018.

Turns out religious tests aren't just unconstitutional. They're also historically unpopular in a democratic republic that prides itself on defending freedom of conscience. At least two GOP senate challengers, Reps. Luke Messer and Todd Rokta of Indiana, have already made an issue of the Durbin-Feinstein bigotry.

"Today, I am calling on Sen. [Joe] Donnelly to return the campaign contributions from Sen. Feinstein and Sen. Durbin, and pledge to not take any more money or attend any fundraisers with Feinstein and Durbin until they both apologize," Messer said Monday. "Bigotry targeted at someone's religion is beyond the pale."

Donnelly withdrew his opposition to the Catholic judge but not before Rokita also slammed him for "saying one thing in Indiana and doing another in Washington D.C."

For Donnelly, the attack could prove especially damning. He's an Irish Catholic representing a fairly religious state, and hailing from its most Catholic corner, where Barrett happens to teach law. And he's taken cash from both Feinstein and Durbin. The aftermath of their unholy inquisition could haunt his campaign.

Other Catholic Democrats will have to pay penance to lesser degrees. Sens. Heidi Heitkamp of North Dakota and Joe Manchin of West Virginia will probably find the attacks annoying. Sens. Claire McCaskill of Missouri and Bob Casey of Pennsylvania could find themselves on the rack. Either way, religion will play a more significant role in every Catholic Democrat's re-election.

Voters won't worry if their senators are beholden to Rome. They will wonder if incumbents believe in the bigotry of Durbin and Feinstein.

Note: This story has been updated to include a statement from Rep. Todd Rokita, who along with Rep. Luke Messer is also seeking the GOP Senate nomination.

Philip Wegmann is a commentary writer for the Washington Examiner.