According to two very non-primary references, the President called Democrats "prison vermin".
The first is the 1902 Political X-rays, saying that it is quoting from an 8 November 1901 letter to the editor of the Herald, and specifying the location as Paris:
Roosevelt has described Democrats as "prison vermin." There are over six millions of us as voters, and if we are not all politically pure as "Jim" Blaine , "Matt" Quay and "Tom" Platt , those of us who are fighting for "purer manners and better laws"
and the second is the 1909 Rooseveltiana by Leslie Chase and seems to quote from a 29 March 1898 letter to the editor of the New York Herald, and says the author of the letter is also Leslie Chase:
Mr. Roosevelt has called Democrats "prison vermin." Yet there are over six millions of us, and those of us still out of jail next November will vote for Mr. Parker. Now the English press is shrieking out with mad delight...
(However, the 1898 date doesn't make sense because Roosevelt didn't run against Parker until 1904.)
It it true that Roosevelt referred to Democrats as "prison vermin"?