Unions have been fighting for a change to the law that would allow health plans for union members to be subsidized, something that is now not allowed under the law. Cruz said that would likely be Obama's next step.

"I'll make a prediction," he said on the Senate floor. "If the Senate doesn't act now, if it doesn't defund ObamaCare, if it doesn't stand up and stop, this before President Obama leaves the White House, he will grant an exemption to those union bosses.

"It's the trifecta of the privileged classes being exempted," he added, noting carve-outs for companies and members of Congress.

Cruz made his prediction near the end of his 20th hour on the Senate floor.