I know it is kind of late, but here is some coverage of the 1948 DNC from WPIX-TV:
https://www.youtube.com/watch?v=tas3kJkFKiEAlso, this TV special aired April 30, 1949, celebrating the tenth anniversary of the launch of TV broadcasting in the US has some fragments from NBC's coverage of the 1940 or 1944 election night around the 12-minute mark:
https://www.youtube.com/watch?v=r5qR5p8oPssHad the kinescope process for recording TV broadcasts been developed about 8 years earlier, then there would have been a lot more surviving footage from the 1940 and 1944 election night coverage.