Tuesday night’s debate between President Barack Obama and Republican Mitt Romney attracted an average of 37.4 million viewers in its first hour on the four major broadcast networks.

Comcast Corp. NBC led the broadcasters, according to preliminary Nielsen data Wednesday from the networks. The early numbers are subject to revision, and don’t include cable channels such as CNN and MSNBC. Nielsen will release a total including cable later today.

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President Obama, seeking to recharge his candidacy, made a spirited defense of his record as he challenged Romney Wednesday night on taxes, immigration and Libya. Obama branded Romney’s fiscal plans “sketchy” and his social policies “extreme,” as he worked to put the former Massachusetts governor on the defensive three weeks from Election Day.

The first debate between the candidates attracted an audience of 67.2 million, counting broadcast and cable outlets, while the faceoff between Vice President Joe Biden and Republican challenger Paul Ryan drew 51.4 million viewers.

Obama and Romney meet for a final debate at 9 p.m. New York time on Oct. 22 at Lynn University in Boca Raton, Florida, where they are scheduled to discuss foreign policy. Bob Schieffer, host of “Face the Nation” on CBS, will moderate.