Congressional candidates and congressmen need lawyers. In D.C., law firms are often lobbying firms. That can create interesting dynamics.
For instance: Rep. Michael Grimm, R-N.Y., recently famous for threatening violence against a reporter, is under scrutiny for campaign finance matters. His recent campaign finance filings show he owes more than $416,000 in legal fees to a law firm (h/t Dave Levinthal). That firm happens to be K Street kingpin Patton Boggs.
Grimm sits on the Financial Services Committee. The firm to which he is indebted represents Citigroup, Visa, AIG, Prudential, Goldman Sachs, and many others under his committee's jurisdiction.