I think in the case of teachers’ unions, there’s a clear quid pro quo. Democrats oppose education reform initiatives that could harm teachers’ unions but help students. Just ask Michelle Rhee about her experience in DC. She’s about as liberal a Democrat as they come, but not beholden to unions.
On Politically Incorrect, Willie Brown (former Mayor of San Francisco and California Assemblyman), was once asked how he could take money from tobacco companies, when he had consistently voted for stricter anti-smoking laws. He replied that if you can’t take money from people and then screw them over, you don’t deserve to be in politics.
You can ask for and receive donations as long as there is no quid pro quo. It’s not bribery unless the money is being paid in exchange for specific actions.
Oh, spoiler virgin, don’t ever change.