Florida Senate race suddenly looks risky for Marco Rubio

Democrat Val Demings, once an underdog in the Florida race for the U.S. Senate, is closing in on incumbent Sen. Marco Rubio, putting into play a seat that Republicans once considered relatively safe as they battle for the majority.

Polls put Mrs. Demings, who represents the state’s 10th Congressional District, within 3 percentage points of Mr. Rubio, a Republican running for a third term. The race is starting to look like a toss-up.

While Mrs. Demings saturates the state’s major media markets with advertisements promoting herself as a tough-on-crime lawmaker who served as Orlando’s...