House Minority Leader Nancy Pelosi said in May that the Democratic Party should not require its candidates to support the right to an abortion. “This is not a rubber-stamp party,” she told the Washington Post. Senate Minority Leader Chuck Schumer agreed.

In July Rep. Ben Ray Luján of New Mexico, the head of the Democratic Congressional Campaign Committee, told the Hill he would be open to funding pro-life candidates in 2018 House races. California Gov. Jerry Brown said on “Meet the Press” that candidates’ positions on abortion “should not be the basis for their exclusion.”

We’ve heard this talk before about a “big tent” party. What’s new is the sudden emphasis on accepting antiabortion Democrats as candidates and funding their campaigns. Last year this was not in Democrats’ plans. The Democratic platform took a rigid pro-choice position that called for taxpayer funding of abortions. The advocacy group Naral Pro-Choice America was thrilled, calling it “the best ever for reproductive freedom.”

That was before Donald Trump had won the presidency with a boost from traditionally Democratic voters. It turned out they were attracted more by his conservative stances on social issues like abortion than by his economic views, a poll by the Voter Study Group discovered. Mr. Trump wound up capturing five Midwestern states—Iowa, Michigan, Ohio, Pennsylvania and Wisconsin—where social issues resonated. Barack Obama won each of those states twice.

The Democrats’ recent step to the right on abortion is small but significant. It shows party leaders have awakened to their weakness on social issues. “Pelosi knows that if Democrats run fanatical pro-abortion candidates in purple districts, they’re not going to win the House” in 2018, says Jeff Bell, a Republican strategist at the American Principles Project.