An anagram of the word “senator” is the word “treason.” This anagram has been noted since at least 1951, when it was printed in the New Statesman and Nation.
 
“An anagram for senator is treason” was posted on Reddit—Shower Thoughts on August 17, 2017. “‘Senator’ is an anagram for ‘Treason’” was posted on Reddit—Shower Thoughts on June 16, 2018.
   
         
Google Books
17 February 1951, New Statesman and Nation, “Funny, Peculiar or Ha-ha” by Robert Armstrong, pg. 200, col. 3:
Who discovered the CARTHORSE in the ORCHESTRA; the GARDENIA in the DRAINAGE; the TREASON in the SENATOR ?
 
Google Books   
New York
Volume 8
1975
Pg. 102:
TREASON: SENATOR BLAMED. BEDLAM!
Dr. Harry W. Hazard, Rochester. N.Y.
 
Google Groups: alt.anagrams
What is a Senator?
Jon Gearhart
10/1/00
Yes.  Senator=Treason is a moldie oldie…
 
Representatives=
Perverse in taste
Severe penis tart
Viper near testes… [They are pricks?]
     
Twitter
Chris Sadler
@cdsadler
senator anagram? treason. huh.
1:21 PM - 17 Oct 2008
   
Twitter
Nolan Bailey
@NolanBailey
the anagram for treason is senator.
9:25 PM - 1 Apr 2009
   
Twitter
Woodie1942
@Woodie1942
Note: Senator is the anagram for treason
9:13 PM - 7 Jun 2009
 
Twitter
David Knight
@libertytarian
Replying to @DRUDGE
@DRUDGE TREASON is an anagram of SENATOR.  True once again
3:05 PM - 23 May 2015
 
Reddit—Shower Thoughts
An anagram for senator is treason.
submitted August 17, 2017 by aidissonance
   
Reddit—Prequel Memes
When you realize treason is an anagram of senator (i.redd.it)
submitted December 27, 2017 by ffuentesbot
   
Twitter
thank you, corporations
@punished_picnic
you: senator is an anagram of treason
me: assassin is an anagram of ass in ass
2:54 PM - 17 Jan 2018
 
Reddit—Shower Thoughts
‘Senator’ is an anagram for ‘Treason’.
submitted June 16, 2018 by mistercathead