NEW YORK (REUTERS) - About 35.6 million Americans watched the US vice-presidential debate between Republican Mike Pence and Democrat Tim Kaine on television, according to preliminary data on Wednesday, about half the number who tuned in to Donald Trump's first matchup with Hillary Clinton last week.

Broadcasters CNN, Fox and NBC, citing ratings data from Nielsen, said some 21 million people watched Tuesday's 90 minute encounter on the four main U.S. broadcast networks, with another 14 million watching on cable channels.

Final figures, including the audience on PBS and smaller networks, are expected later on Wednesday but will break no records.

Last week's first debate between the presidential candidates for the Nov 8 election, Republican Trump and Democrat Clinton, was watched by a record 84 million TV viewers.

Vice-presidential debates have traditionally attracted smaller TV audiences, with the exception of the 2008 encounter between Democrat Joe Biden and Republican Sarah Palin, which set a record for a VP debate of 69.9 million viewers.

The 2012 vice-presidential encounter between Biden and Republican Paul Ryan was watched by 51.4 million Americans.

None of the figures for the debates include those who watched online, through social media, or in bars and restaurants.

Tuesday's 90-minute face-off between Pence, governor of Indiana, and Kaine, a US senator, was the only debate between the vice presidential contenders before the election next month.

Among the networks, NBC had the biggest audience with an average of seven million viewers, followed by Fox News Channel with 6.1 million.