I think King would be more likely to run as an Independent in a scenario where Warren or Sanders is the Democratic nominee than he would be actually running as the Democratic nominee, but what do I know.
If Angus King runs as a Democrat:
Senator Angus King (I/D-ME)/Senator Kamala Harris (D-CA) 49%
POTUS Donald Trump(R-FL/NY)/VPOTUS Mike Pence (R-IN) 46%
If King runs as an Independent (presumably because Sanders is the Democratic nominee):
POTUS Donald Trump (R-FL/NY)/VPOTUS Mike Pence (R-IN) 41%
Senator Bernie Sanders (I/D-VT) Vermont/Senator Tammy Baldwin (D-WI) 29%
Senator Angus King (I/D-ME)/Senator Lisa Murkowski (R-AK) 27%
