He spoke briefly Monday morning at a press conference organized by his supporters.
The "historic" vote on the House floor is set for Thursday afternoon.
The Senate previously passed its omnibus public safety bill without police accountability legislation.
The bill would allow people 21 and older to use marijuana legally, with restrictions.
"Those who made threats of violence at the rally in St. Paul should be investigated and held accountable," the letter states.
House Majority Leader Ryan Winkler said if they don't denounce the threats they "need to be held accountable."
Zerwas spent time in an ICU ward receiving supplemental oxygen.
Thompson made strong, divisive remarks outside Lt. Bob Kroll's home in Hugo last month, leading to calls for the DFL to drop its support for the candidate.
The state lawmaker went viral earlier in the pandemic due to comments he made about death certificates.