LOS ANGELES (Reuters) - Barack Obama’s campaign “infomercial” was the most-watched telecast in U.S. prime time on Wednesday, drawing an “American Idol”-size audience that easily eclipsed even the climax to baseball’s World Series.

More than 33.5 million viewers tuned in to watch the Democratic presidential nominee’s paid 30-minute message, aired on three major broadcast networks and four smaller channels, Nielsen Media Research reported on Thursday.

The three big networks alone, CBS, NBC and Fox, accounted for 25.5 million viewers combined -- 1.2 million more than they drew in the same half hour a week ago, Nielsen said.

By comparison, 19.8 million viewers watched the conclusion to baseball’s World Series championship as the Philadelphia Phillies defeated the Tampa Bay Rays. The game, ranking as the night’s No. 2 broadcast, was carried by Fox following Obama’s multi-network commercial.

Fox likewise is home to U.S. television’s No. 1 series, “American Idol,” which averaged more than 28 million viewers weekly last season.

But the Illinois senator’s infomercial audience was dwarfed by ratings for the three recent debates between Obama and Republican rival John McCain, which averaged 57.4 million viewers on 11 networks. More than 38 million tuned in to see Obama’s speech accepting the Democratic nomination in August.

The infomercial blended pre-taped segments of Obama directly addressing viewers with a video montage of the nominee on the campaign trail and ordinary Americans talking about their economic struggles. Opening with a shot of windblown fields of grain, it concluded with Obama addressing a live campaign rally in Florida.

TV critics gave generally positive reviews of the ad, with The Washington Post’s Tom Shales calling it reminiscent in tone and texture of the “Morning in America” campaign ad produced for President Ronald Reagan’s re-election bid in 1984.

The Obama piece was a throwback to a political advertising format that was common in the 1950s and ‘60s.

It marked the first such paid national political telecast since independent candidate Ross Perot ran one on the eve of the 1996 election, drawing nearly 22.7 million viewers on three networks. A series of similar ads supporting his 1992 White House bid averaged 11.6 million viewers.

Obama’s campaign paid an estimated $1 million per network for its 30-minute spot, which gave both CBS and NBC a double-digit ratings boost over their regular series programing in the same half hour a week ago.

Obama’s guest appearance later that night on the “The Daily Show with Jon Stewart” likewise lifted that Comedy Central cable network program to its biggest audience ever -- 3.6 million viewers.