http://dealwithit.net/

They will send this letter through the site for you:

Subject:
Do Not Vote For The Senate Immigration Reform Plan!
To:
Senators Kyl, Kennedy & McCain
Solution:
Dear Senator,

If you vote in favor of the new Senate Immigration Reform Plan, I will support for the rest of your political career every opponent that runs against you and will fund your recall drive.

As an active citizen, I will be closely monitoring your vote and will know by your decision which way to invest my time and resources.

Sincerely,