He can resign next March and start fundraising. And I assume that he has cash in his old House acampaign account that he could transfer to a Senate run. Besides, most of the money will be spent by the NRSC and by conservative PACs, which can start running “issue ads” pointing out Jon Tester’s liberal votes well before Zinke announces. (The NRSC only has two potentially vulnerable GOP incumbents that it will need to defend, so it will have plenty of cash to spend softening up support for RAT incumbents such as Tester, Heidi Heitkamp, Joe Manchin, Claire McCaskill, Joe Donnelly, Tammy Baldwin, Sherrod Brown, Debbie Stabenow, Bill Nelson, Amy Klobuchar, Tim Kaine and Angus King(and (and maybe Martin Heinrich and Bob Menendez).
The question is whether he’s even going to run, and I’ve heard and seen nothing to indicate that he actually will since leaving the House. It seems like a waste of time and effort to have gone through the confirmation process for Interior Secretary only to hold the position for a year (or less) if his sights were already set on running for the Senate seat, for which then holding the Cabinet office would interrupt badly needed time to fundraise.