There was a threat about this a few months ago:
https://uselectionatlas.org/FORUM/index.php?topic=188084.0But hey, it's a topic I still love to discuss so I wont shut it down.
Democrats in 2020:
Julian Castro
Joaquin Castro
Bill De Blasio
Kristen Gilbrand
Martin Heinrich
Martin O'Malley (Assuming he doesn't get the 16' nomination)
Corey Booker
Andrew Cuomo
Kamala Harris
Republicans in 2020:
Tom Cotton
Marco Rubio (Assuming he doesn't get the nomination in 16')
Rand Paul (Assuming he doesn't get the nomination in 16')
Scott Walker (Assuming he doesn't get the nomination in 16')
Ted Cruz (Assuming he doesn't get the nomination in 16')
Kelly Ayotte
Paul Ryan
Brian Sandoval