Lawyers don't exchange their views as private citizens for a license to practice law. However, they do need to be careful when it comes to advocating to lawmakers. The line between lawyer and lobbyist is a fine one, and ambiguities can leave everyone wondering whose side you're on in any given moment. Just look at the confusion swirling around Rudy Giuliani.
A lobbyist's activities are subject to federal, state, and local laws; meanwhile, attorneys have their own rules of professional responsibility to uphold. Where these rules intersect, is where the lawyer/lobbyist can operate.
Under the Lobbying Disclosure Act, a lobbyist is an individual who:
Whether a lobbyist must register is another question entirely. The Lobbying Disclosure Act allows a lobbying firm to avoid registering for a particular client if the total income for that client doesn't exceed $3,000 per quarter.
The rules of professional responsibility don't address lobbying directly, but state bars often interpret conflict of interest rules and apply them to lobbying activities. Rule 1.7, for example, would prohibit a lawyer from advancing adverse positions in the same lobbying matter. The rules on candor before a tribunal apply as well.
Some states require all lobbyists to register, while others only require registration once a certain compensation threshold is met. The National Conference of State Legislatures provides a helpful chart laying out the lobbyist rules for each state.
Municipal governments can have their own rules for attorney lobbyists as well - for example, San Diego's Lobbying Ordinance. If an attorney from a law firm lobbies the San Diego City Council on behalf of a client in exchange for compensation, the firm must then register as a "Lobbying Firm."
The most important thing is to be upfront about your activities - if others are wondering who you're working for or whose interests you represent, you might be skating too close to the line.