Columbian ground squirrel. Despite the ringed tail, the resemblance (and the fact that two different commenters told me it was a CGS) is conclusive.
My guy on the left, a photo from the web on the right.
Thanks to Erika and that other reader who also identified my rodent as a Columbian ground squirrel. (I can't remember her name and her comment seems to have disappeared, but she is a former MT resident so knows whereof she spoke wrote)