WASHINGTON — Democrats just can’t help themselves. They were winning the battle over family separations at the southern border. Americans of all political persuasions were horrified by the images of children in cages separated from their parents. Despite President Trump’s efforts to blame Democrats for the catastrophe, polls showed that a plurality of Americans placed the blame squarely on the president’s shoulders. Evangelical Christian leaders — who stood with Trump through Stormy Daniels and the “Access Hollywood” tape — began to publicly criticize the family separations. The Rev. Franklin Graham called family separations “disgraceful,” and the Southern Baptist Convention passed a resolution declaring them “inconsistent with the gospel of Jesus Christ.”

But then, Democrats blew it — and gave the president the upper hand once again.

First, they went overboard in their attacks on Trump, with some playing the Hitler card. “This is the United States of America. It isn’t Nazi Germany,” said Sen. Dianne Feinstein, D-Calif., the ranking member of the Senate Judiciary Committee. Sorry, senator, there are no gas chambers on the southern border. Even Americans who oppose family separations are repulsed by those who compare Trump to Adolf Hitler and detention centers along the U.S.-Mexico border to Nazi concentration camps.

Then, to compound her error, Feinstein introduced disastrous legislation — now co-sponsored by every Democratic senator — that would not simply end family separations but also actually expand the policy of “catch and release.” (A 2015 court ruling mandated that children who enter the country illegally must be released within 20 days. Feinstein’s bill — which states “in general, there is a presumption that detention is not in the best interests of families and children” — would, by ending family separations, effectively require the parents be released as well).

Democrats don’t seem to understand: Americans oppose family separation, but they also oppose “catch and release.” A new Economist/YouGov poll showed that 54 percent of Americans disapprove of separating families who cross the border illegally. But only 19 percent support “releasing the families and having them report back for an immigration hearing at a later date” — the approach now endorsed by every single Senate Democrat. By contrast, the poll found the most popular policy — supported by 44 percent of Americans and even 49 percent of Democrats — is “holding families together in detention centers until an immigration hearing at a later date.” And it found that 46 percent also support Trump’s “zero tolerance” policy of arresting and prosecuting anyone who crosses the border illegally.

In other words, Feinstein has given away the moral high ground and has put every Senate Democrat in a position of endorsing the least popular option for dealing with families crossing the border illegally.

It gets worse. Feinstein’s bill is so poorly written, it makes no distinction between illegal-immigrant children and U.S. citizens who are under 18 and already in the United States. It applies to not just Border Patrol and immigration officers, but to virtually all federal officers, including FBI and Drug Enforcement Administration agents. It prohibits these federal authorities “from removing a child from his or her parent or legal guardian, at or near the port of entry or within 100 miles of the border of the United States.” This applies to vast swaths of the country. About two-thirds of the U.S. population lives within 100 miles of a U.S. land or coastal border, and many others live in interior cities such as Denver, Nashville and Salt Lake City with international airports that are considered “ports of entry.”

How bad is this? Writing in the Federalist, lawyer Gabriel Malor explains that, under Feinstein’s bill, if the FBI raided the home of a drug dealer in Buffalo and discovered that his minor daughter was with him, the proposed legislation “would prohibit the FBI agents, while arresting a drug trafficker, from separating this child from her father.” Call it the “Catch and Release for Violent Criminals Act.”

This is a political disaster for Democratic senators such as Joe Manchin III, W.Va., Heidi Heitkamp, N.D., Jon Tester, Mont., Claire McCaskill, Mo., and Joe Donnelly, Ind., who are running for reelection in states that Trump won by double digits. Thanks to Feinstein, they are now on the record supporting not only “catch and release” for illegal immigrants with minor children, but for felons with U.S. citizen children who commit crimes having nothing to do with crossing the border illegally.

In their rush to exploit the border crisis, Senate Democrats have managed to take an issue that was toxic for Republicans and turn it into a calamity for themselves. That takes a special kind of political stupidity. The lesson is clear: Don’t compare your opponents to Hitler. And think before you legislate.

Marc Thiessen is a columnist with The Washington Post. Follow him on Twitter, @marcthiessen.