33 Senate seats will be up for grabs in 2020. Among these 33 seats, there are some Senators who might be interested in running for President: Tom Cotton (R-AR), Cory Gardner (R-CO), Joni Ernst (R-IA), Gary Peters (D-MI), Al Franken (D-MN), Ben Sasse (R-NE), Cory Booker (R-NJ), Mark Warner (D-VA), Shelley Moore Capito (R-WV).
Back in November, an Arkansas lawmaker proposed a bill that would allow candidates to seek multiple federal offices in the same election. I don't know what's the status of that bill right now.
http://www.huffingtonpost.com/2015/03/11/tom-cotton-president-2020_n_6849064.html
