Rep.-elect Alexandria Ocasio-Cortez arrived on Capitol Hill last month and promptly joined a climate-change sit-in at the office of House Minority Leader Nancy Pelosi. Yet the Democratic Socialist freshman from New York soon endorsed Mrs. Pelosi for speaker. So did the Democratic caucus, by a vote of 203-32.

Don’t be fooled by this show of party unity. Progressives don’t trust Democratic leaders. For evidence, look to a down-ballot leadership race. Hakeem Jeffries of New York defeated Barbara Lee of California for party caucus...