The Stop Trump forces are beginning to pour money into television ads, with a particular focus on the big states voting on March 15. Four different groups have reserved at least $10 million in airtime in Florida so far, according to trackers of media spending. That number is expected to grow, but television stations in Florida are already awash in such ads.

Two from the American Future Fund, which has spent $2 million so far in Florida and Illinois, show decorated veterans assailing Mr. Trump as a poseur on military matters. Michael Waltz, a retired Special Forces colonel, blisteringly calls Mr. Trump a draft dodger and, effectively, a coward. “Donald Trump hasn’t served this country a day in his life,” he says. “Don’t let Trump fool you.”

And a former prisoner of war in Vietnam, Tom Hanton, bluntly questions Mr. Trump’s toughness: “Trump would not have survived the P.O.W. experience. He would have been probably the first one to fold.”

Separately, Club for Growth Action, an arm of the anti-tax group that was the first to run ads in Iowa against Mr. Trump, has placed $2 million in commercials attacking him in Illinois on top of $1 million in Florida.