Democrats have finally figured out how to turn Trump’s tweets into their own weapon.

The Almanac of American Politics details Sen. Gillibrand’s eye-rolling flip-flops—famous in New York political circles—from upstate House conservative to progressive Senate saint, described in an apparently forgotten New York Times account.

.. With Democrats themselves admitting they have no coherent message that could win a presidential election, the opposition strategy has been built around Mr. Trump’s personality, his alleged collusion with Russia to disable Hillary Clinton, and now the return of the same accusations of sexual harassment that did not cause him to lose the election.

.. Democrats may finally have hit upon the Achilles’ heel that will fell or weaken this president: his tweets.

The tweets have worried Republicans and Trump supporters since they started. Mr. Trump rejects this criticism. He said, with a tweet, that they energize his base. But Roy Moore just lost in Alabama.

.. The biggest nonpolitical story for months has been sexual abuse, starting with Harvey Weinstein. It was only a matter of time before the politicians would figure out how to manipulate harassment for their own purposes.

.. Then on Monday, the day before the Alabama election, came the following: Three women repeated sexual harassment accusations they’d made against Mr. Trump during the campaign; the congressional Democratic Women’s Working Group called for an investigation of the charges; and Sen. Gillibrand called on the president to resign.

.. They have discovered how to make his tweets their weapon.

.. Mr. Trump’s Tuesday-morning tweet suddenly elevated a B-level New York senator, and the media instantly recycled the Trump sexual-harassment details. Doug Jones defeated Roy Moore by just 1.5%, and the Republicans’ Senate majority fell to 51. By day’s end, Sen. Gillibrand was soliciting funds via email for her 2018 election. They figured out how to make the Trump side lose. It’s the president’s move now. Checkmate awaits.