WASHINGTON — The Democratic Congressional Campaign Committee launched radio and Web ads yesterday against 21 House Republicans they consider vulnerable over the issue of middle-class tax cuts.

The commercials criticize members who have refused to sign a petition to bring a bill to the floor that would prevent tax increases on everyone making less than $250,000.

The Senate has already passed the measure, and House Democrats will need 218 votes to clear it in their chamber.

Republicans hold a majority in the House, 241-192.

The ads follow similar spots taken out earlier in the week by the conservative group Crossroads GPS, which targeted six Democrats, encouraging more spending cuts.