(And "millionaire" (net worth >$1M) isn't really a terribly high bar for mid- to late-career professionals.) That's not to say they're representative of their average constituents' income levels but I wouldn't reasonably expect them to be.
Also you don't get a million in debt by being poor.
http://ballotpedia.org/Net_worth_of_United_States_Senators_a... lists median net worth over a million\
http://www.opensecrets.org/news/2014/01/millionaires-club-fo... also puts median net worth over 1 million
http://time.com/373/congress-is-now-mostly-a-millionaires-cl... also lists median net worth over a million
http://money.cnn.com/2015/01/12/news/economy/congress-wealth... also puts median net worth above 1 million
http://www.upi.com/Top_News/US/2015/01/13/Congress-worth-43B... also puts median net worth over a million