At least 40 million Americans watched Senator Barack Obama accept the Democratic nomination for president Thursday night, a record for convention viewership that exceeded even the expectations of his aides.
The historic speech by the first African-American presidential nominee of a major political party reached 38.4 million viewers on 10 broadcast and cable networks, Nielsen Media Research said Friday. PBS estimated that an additional 3.5 million had watched its prime-time coverage.
The ratings dwarfed the audience for the Summer Olympics and the season finale of "American Idol" in May, and added to what was already a sense of buoyancy within the Obama campaign that the night had gone better than planned.
Despite Republicans' advance ridicule of the enormous venue, Invesco Field, and the set, an elaborate, columned backdrop, Democrats went to bed having heard terrific reviews of the final night of their convention. Indeed, the backdrop, initially derided as resembling a Greek temple — playing into the Republican line of attack that Mr. Obama's supporters had deified him — turned out to be something of a hit; television reviewers and commentators praised the overall staging.
"The stagecraft was so phenomenal," Andrea Mitchell said on MSNBC, adding, "I don't know how they could have done it any better."
The four-night convention was the most-watched since 1960, when Nielsen began measuring the events. The 10 p.m. hour, Eastern time, from Monday to Thursday was viewed by an average of 22.4 million households, Nielsen said, surpassing by half a million the Republican convention of 1976, previously top-rated.
The comparisons with prior conventions come with a number of caveats: convention coverage is shown on more channels now, and the coverage is shorter, at least on the broadcast networks.
Regardless, Thursday night's record was surely impressive. The television audience for Mr. Obama's speech was half again as large as the viewership for the acceptance speeches by President Bush and Senator John Kerry in 2004.
"Obama had an opportunity to get his message across to a record-breaking crowd of millions of American voters, and he used it effectively," said Bill Burton, a spokesman for the campaign.
Demonstrating the gradual shift in the political news audience from broadcast to cable, CNN attracted more viewers than any of the broadcast networks during the 10 p.m. hour on Wednesday and Thursday. (Fox News Channel defeated the broadcasters during the Republican convention in 2004.) Mr. Obama's speech, which he gave in that hour, reached 8.1 million viewers on CNN, 6.6 million on ABC, 6.1 million on NBC, 4.7 million on CBS, 4.2 million on Fox News, and 4.1 million on MSNBC. Other viewers watched on additional channels.
Jonathan Klein, the president of CNN/U.S., said the high cable ratings were partly a result of 18 months of intense campaign coverage.
"We've approached the election like a very long baseball season," Mr. Klein said. "We've developed relationships with viewers, so they know they can depend on us."
Mr. Obama's speech drew an especially high number of African-American viewers. Excluding sporting events, Nielsen said, the speech ranked second in black viewership among all programs over the last decade. Only a Michael Jackson special in 2001 did better.
After an initial burst of coverage on the network morning programs, the images from Thursday night were quickly overwhelmed by conflicting reports about Senator John McCain's likely running mate that completely took over news coverage on cable.
That sort of repetition from the previous night is ordinarily what presidential campaigns count on to spread the better moments of their nominating conventions to those who did not watch them. But Mr. McCain's unexpected selection of Gov. Sarah Palin of Alaska left no room for a look back at the night that had passed, although online the speech had been viewed more than 300,000 times on YouTube by Friday evening.