Editor’s note: A longer version of this article first appeared at American Spectator.

Bill and Hillary Clinton have endorsed gay marriage, completely reversing their support of the 1996 Defense of Marriage Act, which defined marriage as between one man and one woman, and was signed into law by President Bill Clinton.

Mrs. Clinton calls herself a “progressive.” It’s funny, I wrote an entire book on Hillary Clinton, and never once heard her call herself a “progressive.”

Well, that’s just as well. The progressive tag fits best. After all, that’s what she and other liberals are doing: they are ever evolving, changing, progressing along to something. Their positions are forever in flux, with the only commonality being that they favor more government centralization to handle perceived injustices. The evolution across issues is so vast, so unceasing, that no progressive can tell you where they will stand years from now. They merely know they’re progressing.

The marriage issue is an excellent case in point. No progressive 100 years ago could have conceived of gay marriage. In fact, merely a decade-and-a-half ago, the entirety of the Democratic Party supported traditional marriage, codified under law. And yet, Democrats turned on a dime in faithful obedience to Barack Obama’s mountaintop-message sanctifying gay marriage a year ago.

Obama promised “change” and “fundamental transformation.” His faithful supporters roared approval, projecting upon his blank screen whatever they had in mind. In Obama’s mind, this included bestowing unto himself the monumental ability to literally redefine marriage, granting himself and his government a power heretofore reserved for the laws of nature and nature’s God.

As for the Clintons, consider their change, their fundamental transformation, their progress on this bedrock issue:

As noted, in 1996, Bill Clinton signed the Defense of Marriage Act. The Arkansas Baptist stood for marriage as always understood.

As for Hillary, the lifelong Methodist was firmly in the camp of not rendering under government the ability to redefine marriage. Her youth pastor and mentor, the Rev. Don Jones, once said: “She is for gay rights…. But I think both she and Bill still think of heterosexuality as normative.”

Yes, they did. Campaigning for the Senate in 2000, Hillary insisted: “Marriage has historic, religious and moral content that goes back to the beginning of time, and I think a marriage is as a marriage has always been, between a man and a woman.” In 2003, she reaffirmed: “marriage … should be kept as it historically has been.” She continued that position throughout the 2008 Democratic primaries.

Alas, jump ahead to last week, where Hillary proudly proclaimed: “LGBT Americans are … full and equal citizens and deserve the rights of citizenship. That includes gay marriage.”

Gee, what happened?

Well, if you’re confused, you need to unravel the illogic of progressive ideology. By progressive thinking, the Hillary and Bill of, say, 5, 10, 15, or 50 years ago were not finished progressing. This should also mean that the Clintons were in fact wrong at each way-station in their journey to today’s progressive “truth” on marriage. Thus, too, it should mean that every Democrat who agreed with them was wrong. Current progressive ideology asserts that only current progressives are currently “right” on marriage.

Are you with me?

But here’s the kicker: How can the Clintons – or any modern progressive – know they’re right now? How do they know they’ve progressed to the “correct” point on marriage? Progress, after all, never stops progressing.

And so, for progressives, where’s their next redefinition in the ongoing process of redefining marriage? Does the evolution end with one man and one woman, or one man and one man, or one woman and one woman? Why could it not next progress to one man and multiple women? Could it involve an adult and a minor? Could their evolving redefinition include first cousins or a parent and child? Could it include multiple heterosexuals or homosexuals in single or even joint or group spousal relationships?

The answer: progressives, by their very definition, cannot answer you.

We do know, however, that progressives are happy to do with marriage what they do with everything: hand it over to the federal government. Render under government what is government’s. And what is government’s province? It’s anything progressives decide.

As for Bill Clinton, who once assured us “the era of big government is over,” he’s on board for the grand project.

Progressives might disagree with conservatives, but at least they know where conservatives stand: we look to tradition, to Biblical law, to Natural Law, to time-tested things worth conserving. We see marriage best as it has been since the Garden of Eden. We can tell you our end-goal, our ideal. Progressives cannot.

And that, ladies and gentlemen, is a train-wreck of an ideology, with literally no end to its havoc. It is currently careening into the most fundamental building block of human civilization: the family.

Dr. Paul Kengor is professor of political science at Grove City College, executive director of The Center for Vision & Values, and New York Times best-selling author of the book, “The Communist: Frank Marshall Davis, The Untold Story of Barack Obama’s Mentor.” His other books include “The Crusader: Ronald Reagan and the Fall of Communism” and “Dupes: How America’s Adversaries Have Manipulated Progressives for a Century.”