WASHINGTON — For decades, Catholic Democratic politicians have been justifying their pro-choice position by telling us that they were personally opposed to abortion but could not impose their religious view on others. For most, the argument was a fig leaf to justify their shameful failure to protect innocent unborn life. But it appeared that Joe Biden really believed it.

For more than 40 years, Biden supported the Hyde Amendment, which bars federal funding for abortions. In 1994, when a constituent wrote to Biden, urging him, “Please don’t force me to pay for abortions against my conscience,” Biden replied, “I agree with you.” He pointed out that he had voted no fewer than 50 times against federal funding of abortions, promising, “Those of us who are opposed to abortion should not be compelled to pay for them.” In his 2007 book “Promises to Keep,” Biden wrote, “I’ve stuck to my middle-of-the-road position on abortion for more than 30 years.”

But middle-of-the-road is no longer good enough in today’s Democratic Party. So when Biden recently reaffirmed his support for the Hyde Amendment, his opponents for the 2020 Democratic presidential nomination pounced. “There is #NoMiddleGround on women’s rights,” Sen. Bernie Sanders, I-Vt., tweeted. “Abortion is a constitutional right.” Sen. Kamala Harris, D-Calif., chastised Biden, declaring “No woman’s access to reproductive health care should be based on how much money she has. We must repeal the Hyde Amendment.” Sen. Elizabeth Warren, D-Mass., and a number of other Democrats also piled on, as did Planned Parenthood, NARAL and Emily’s List.

As recently as last week, the former vice president was still standing firm. It appeared this might be his “Sister Souljah moment,” when he separated himself from the extremists in his party. This was both principled and good politics: Just 36% of Americans support federal funds to pay for abortion.

But then Biden gave in to the mob. He tried to justify his flip-flop at a Democratic National Committee event in Atlanta last Thursday night by declaring, “If I believe health care is a right, as I do, I can no longer support an amendment that makes that right dependent on someone’s ZIP code.” This is absurd. Americans have a constitutional right to keep and bear arms, but the government is not obligated to provide weapons to poor people who cannot afford them.

Biden’s surrender to the pro-abortion radicals damaged his presidential prospects. First, it made him seem weak and unprincipled. Few pro-choice voters would have abandoned Biden because he opposed taxpayer-funded abortion; indeed, plenty of pro-choice voters agree with that position. But voters will abandon a politician who abandons his principles.

Second, it hurt Biden with the one group he claims he can win back for Democrats: working-class voters who cast their ballots twice for him and President Barack Obama, but switched to Donald Trump in 2016. These once-reliable Democratic voters are more socially conservative than the liberal elites.

Third, Biden’s capitulation refocused the national spotlight on abortion extremism in the Democratic Party. After Alabama passed the most restrictive abortion law in the country, Democrats thought they had the upper hand in painting Republicans as abortion extremists. But now the focus is back on Democrats and their insistence on taxpayer-funded abortion-on-demand up to the moment of birth.

Democrats don’t seem to understand that most Americans — including many who identify as pro-choice — don’t see abortion as something to celebrate but as a necessary evil that should be allowed only in some limited circumstances.

Marc Thiessen writes a twice-weekly column for The Post on foreign and domestic policy and contributes to the PostPartisan blog. He is a fellow at the American Enterprise Institute, and the former chief speechwriter for President George W. Bush.