cosponsor

v. support, sponsor in conjunction with other sponsors

n. co-supporter, one of two or more sponsors

Sponsor, co-sponsor, patron, and co-patron are all terms for legislators who jointly introduce a bill. Their names are typically listed on the first page of the bill. It is presumed that, if no one else, at least this core group will vote for the bill. Sometimes a legislator will note that they are a sponsor "by request" when they introduce a bill on behalf of a constituent and wish to distance themselves from it.
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Verb
1. sponsor together with another sponsor
(hypernym) sponsor, patronize, patronise

n. cosponsor

COSPONSOR. COSPONSORIZZARE