The attacks, set to air on cable television and online, are an unusually early effort to nationalize the battle for control of Congress. They target 10 incumbent Republican lawmakers in areas where Mr. Trump is expected to run poorly, including Denver, San Antonio and the Chicago suburbs.

In one of two ads, the Democratic committee accuses Republicans of enabling a “bully” with “ideas that threaten our country’s security.”

“Republicans in Congress are just standing by him,” the ad says. “But shouldn’t they really be standing up to the bully?”