Caitlyn Jenner better reconsider making a run for the U.S. Senate ... at least according to her ex-wife, Linda Thompson.

We got Linda, who was married to Caitlyn for 5 years, outside their son Brandon's Tuesday night gig at L.A.'s Belasco Theater -- and asked her about the political buzz.

Caitlyn said this week she's been mulling over a Senate run, but it's clear Linda won't be voting for her if she does.

She'll happily give CJ a good hearty laugh though!

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