NEW YORK, June 28 (Reuters) -

Over 51 million TV viewers tuned in to watch Thursday's U.S.

presidential debate between Democratic President Joe Biden and

Republican rival Donald Trump, according to final Nielsen data.

The number is about 30% lower than the 73 million people

who watched the candidates' first face-off in 2020, and among

the three lowest-rated first presidential debates since 1976.

The relatively low number compared to past debates in

recent election cycles could be indicative of low voter

enthusiasm for both candidates. It does not capture the full

extent of online viewing, which has grown in popularity as

traditional TV audiences decline.

Media experts were looking to see how a new format by host

CNN would play out, and whether it would provide a template for

future debates. The restrictions of that format - which included

the option for CNN to mute the candidates' microphones - imposed

some discipline on the candidates and should be emulated by

other networks, three media experts said.

CNN, which held the exclusive rights to present the debate,

allowed candidates two minutes for each answer and one minute

for rebuttals, and muted their microphones if they exceeded

those limits. The studio did not have an audience, and

moderators Dana Bash and Jake Tapper did not fact-check the

candidates in real time.

CNN defended itself against the criticism from some media

commentators that the absence of real-time fact-checking allowed

both candidates to spread false claims.

"The role of the moderators is to present the candidates

with questions that are important to American voters and to

facilitate a debate, enabling candidates to make their case and

challenge their opponent," a CNN spokesperson said in a

statement.

Biden delivered a shaky, halting performance while Trump

made a series of exaggerated or false claims that mostly went

unchecked by his opponent.

"Without having an audience, you got a sense of who these

guys are and what they are," said Frank Sesno, professor at the

School of Media and Public Affairs at George Washington

University and a former CNN journalist.

"You got the sense of Trump the attack dog and Biden as

something of the policy wonk and the hesitant old man that you

saw standing behind that mic," Sesno said.

Trump and Biden campaign spokespeople did not respond to

requests for comment about the CNN format and hosts'

performance.

Trump largely refrained from speaking over Biden during the

debate, showing the kind of restraint that eluded him during the

candidates' first debate in 2020.

The debate's format helped "save Trump from some of his

worst impulses," said David Greenberg, professor of history and

of journalism and media studies at Rutgers University, and may

have helped the Republican candidate appeal to voters who have

been put off by his interruptions in previous debates.

"The producers of debates often forget that the main thing

viewers want at a debate is to hear what the candidates have to

say," said Jon Klein, co-founder of Hang Media and the former

president of CNN U.S. "They don't want to see a fight or

performance. The mute button went a long way."

The experts agreed with CNN's decision not to have

moderators fact-check the candidates during the debate, instead

providing a fact-check on its websites and after the debate.

"There were a ton of inaccuracies that were left hanging but

it would have been a mess to fact check the debate in real

time," Sesno said.

CNN provided a feed of its production to other networks and

digital outlets, requiring them to use CNN in the programming

title and keeping the CNN logo on screen throughout the debate.

The debate ratings figure reflects viewers across 16

networks, including CNN, Fox News and MSNBC.

CNN said that the match-up was its biggest debate ever

across its websites, mobile apps and YouTube player.

The next Biden-Trump debate is scheduled to take place on

Sept. 10, hosted by ABC News.