Wait would it be DiFi if the Democrats win the Senate next year then?
Only if something happened to Leahy. I think he's already said he's running for reelection next year, hasn't he? If he doesn't retire until 2022, then that's the earliest that you'd have someone else as the most senior Dem.
Also, if Collins can somehow manage to keep getting reelected in Maine, then she could easily be Pro Tem some day as well (though perhaps not until the 2030s). All of the Republicans more senior than her are at least five years older.
Collins can have her Senate seat for as long as she wants it. Even in the democratic wave of 2008, she won with 60% of the vote against a top-tier recruit. She's completely unbeatable.
