That might leave Bullock running firmly in the centrist, establishment lane of the primary race. And that puts him head-to-head with Joe Biden. Good luck with that fight, Steve.
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A backup in that lane in case Biden flames out?
So who are the Democrats left with if and when Biden tanks who has a prayer of not losing the general election by a landslide? The closest thing to another "centrist" in the race is Klobuchar, and she isn't getting any traction now and is unlikely to do so in the future. They need someone with a modicum of name recognition to fill this niche - though I doubt Bullock will be any more successful at it than Klobuchar.
He is term limited out after 2020 and would like another government job. He would be a longshot to take on Steve Daines, the GOP incumbent up in 2020 for the U.S. Senate. Daines was elected with an 18% margin in 2014.
The other U.S. Senator, John Tester is not up until 2024 and it is no sure thing that he would retire to make room for his fellow rat.
So there is really no downside of Bullock jumping into the ring at this point.