Sen. Joe Lieberman (I-Conn.) said the administration may have problems getting the START treaty signed last week ratified in the Senate.
Lieberman said he'd arrived at his belief on the vote tally falling short after conversations with colleagues over the congressional recess.
"We need that defense to protect our allies and ourselves from Iran," Lieberman said.
President Barack Obama backed away from the controversial missile shield, a plan launched under President George W. Bush, at the beginning of his term, earning kudos from Russia but disappointment from Poland and the Czech Republic.
Lieberman stressed that as stockpiles are slashed, "we have to make darn sure that our nuclear warheads are capable, are modern. And a lot of them are decades old."
"Fox News Sunday" host Chris Wallace asked Sen. Lamar Alexander (R-Tenn.) about the chances of getting nine Republican votes need in the Senate to ratify the treaty.
That depends on the administration's answers to the concerns posed by those like Lieberman, Alexander said.
"I mean, reducing the number of nuclear weapons that are deployed to 1,500 gives us plenty to blow everybody to kingdom come if that's what we choose to do," Alexander said. "But the questions are some of the ones mentioned by Senator Lieberman, and we need to take plenty of time to answer them."
Alexander also took issue with Obama's "troublesome" Nuclear Posture Statement, which declared that the U.S. wouldn't hit a non-nuclear country with nuclear weapons.
"It takes away the ambiguity about our use of nuclear power," Alexander said. "Ambiguity in foreign policy is sometimes very useful, as we've found."