Getty Images

President-elect Donald Trump is meeting with Sen. Heidi Heitkamp (D-N.D.) on Friday, and sources tell Reuters and CNN that Trump is considering the red-state Democrat for either energy secretary or secretary of the interior. Trump is also thinking about tapping Sen. Joe Manchin (D-W.V.) for energy secretary, three sources tell Politico. Heitkamp's Trump Tower meeting in particular has Senate Democrats alarmed, since if Heitkamp joined Trump's Cabinet, North Dakota voters would likely elect a Republican to replace her.

Right now, the Senate has 51 Republicans and 48 Democrats (including two independents), and a runoff election in Louisiana is expected to deliver another Republican. Neither Heitkamp nor Manchin ruled out serving in the Trump administration. Both senators are up for re-election in 2018, expected to be a tough cycle for Senate Democrats, and having incumbents run would significantly improve the Democrats' chances of holding those seats.