It was impossible not to notice that Nancy Pelosi spent President Trump’s 70-minute State of the Union speech grimly chewing her cheek. She was thinking: “What I know, and he doesn’t know, is that history says a year from now I will be speaker of the House, and he’ll be on the brink of impeachment.”

Odds are, she’s right.

Mr. Trump gave a skillfully designed speech. He adroitly wove his conservative policies inside an attractive tapestry of generally self-evident American truths, while extending a fist of compromise to his opponents. But history records that the party of first-term U.S. presidents loses House seats in the midterm elections. If Republicans suffer a net loss of 24 seats, Mr. Trump will fall into the hands of Mrs. Pelosi’s Madame Defarge (keyword: guillotine).

In his State of the Union, Mr. Trump loaded up his presidency on the strong economy, jobs and low unemployment. “It’s the economy, stupid” is a good political bet, but it isn’t decisive.

Lyndon B. Johnson’s midterm election, in 1966, came amid a 3.8% unemployment rate delivered by the 1964 tax cuts. LBJ lost 47 House seats. George W. Bush had an unemployment rate of 4.6% in 2006 and lost 30 seats. For both LBJ and W, the effects of prolonged war—Vietnam and Iraq—undermined a strong economy. The absence of a big war is a plus for Mr. Trump.