How about an email to all your users telling them if the like online gambling to email their senators.
Something along the lines of:
I am strongly against the passing of HR 4411 as I value freedom within my home. I am a member of therx.com. If a vote regarding this bill occurs, therx will email me regarding your position. If you vote in favor of this bill, I will vote for your opponent and encourage all of my friends and colleagues to do the same should you seek reelection.
-Sean