Just hours after the Supreme Court struck down an Arizona law that required voters to prove their citizenship in lieu of federal forms, Senator Ted Cruz (R., TX) and Rand Paul (R., KY) promised to tack an amendment to the immigration bill currently traveling through the Senate. Given that it's such a controversial and difficult issue to address on both sides, any change to the measure at this point could be fatal. Late in the day, Sen. Paul announced his "Secure the Vote" amendment, which provides more checks that individuals are citizens before they are allowed to vote. The amendment has the potential to derail the legislation if accepted into the bill.
Update: Senator Cruz has now filed an amendment with Senator Vitter (R., LA) attempting to negate the preemption doctrine that national laws have precedence over state laws in terms of voting registration.