Yes, ‘Congress’ is a proper noun.
A proper noun is a specific name given to individual entities, including names of people, places, organizations, and sometimes ideas. In the United States, ‘Congress’ refers specifically to the national legislative body of the country, comprising the Senate and the House of Representatives. Because it identifies this particular institution uniquely, it is capitalized. This distinguishes it from common nouns, which refer to general items or concepts. For example, the term ‘congress’ (lowercase) can refer generically to a gathering or assembly, but when used to signify the United States Congress, it is treated as a proper noun.