Senate Minority Whip Jon Kyl (R-Ariz.) and Sen. Joe Lieberman (ID-Conn.) reiterated in separate interviews Monday that ratification of the Strategic Arms Reduction Treaty hinges largely on whether the administration submits an adequate plan to modernize existing nuclear weapons.

President Barack Obama signed the START nuclear weapons reduction treaty with Russia last week and has said he would like the Senate to ratify the document by year’s end. But Senate Republicans and Lieberman are vowing to vote against ratification unless they are satisfied with the Obama administration’s plan to modernize the nation’s current nuclear weapons arsenal.

“I think a lot of Senators might be inclined to support the START treaty, but not without an adequate modernization plan and action on that plan to demonstrate a commitment to it,” Kyl told reporters Monday afternoon. “The two critical areas right now, as far as I’m concerned, are, what kind of funding are you going to commit to, and what limitations are you going to put on the replacement, reuse and refurbishment of the warheads.”

Kyl added, “I have not seen a sufficient commitment out of the Obama administration yet.”

In the late 1990s, Kyl helped defeat the ratification of the comprehensive test ban treaty signed by President Bill Clinton.

Lieberman, a foreign policy hawk, was more diplomatic in his assessment of START’s prospects for ratification. But he was no less adamant that his concerns be addressed. Lieberman said he received a phone call Monday morning and discussed this issue with Secretary of State Hillary Rodham Clinton. Lieberman said the secretary told him she would be able to sufficiently address his concerns.

“Most of us, including me, and others that I’ve talked to, would like to vote for the treaty. So, we’re not looking to stop it. But I do think that we want some reassurances — some specific reassurances … on those two questions I mentioned that we’re going to modernize our nuclear stockpile and that there’s nothing in this that will at all inhibit the development of our missile defense in Europe,” Lieberman said. “If the administration can deal with these concerns … then it’s possible we could ratify it this year.”

The ratification of START is linked by law to the implementation of a nuclear weapons modernization program. Kyl made it clear that the details of any such plan — which has yet to be presented for Congressional review — would determine the treaty’s fate.

The Senate is charged by the Constitution with ratifying treaties. A two-thirds vote is required, meaning that with the chamber’s current breakdown, eight Republicans would have to back START if the Democratic Conference is unified. However, the House will need to appropriate the funds for the nuclear weapons modernization plan that is linked to the treaty, and this could complicate the politics of ratification.

The Obama administration’s Nuclear Posture Review is also affecting the calculus of support for START, with many Republicans concerned that it too severely limits U.S. strategic advantage with respect to how and when nuclear weapons are used. The president this week is hosting a summit of world leaders on the subject of stopping the proliferation of nuclear weapons.

The treaty is not expected to be formally submitted to the Senate until early May.

Should the Republicans’ policy concerns be addressed, Kyl said the ability of START to be voted on this year would depend on how the Democrats approach the financial regulatory reform bill, the nomination of a new Supreme Court justice and other matters. The Minority Whip said the details of the nuclear weapons modernization program would also weigh on START’s schedule.

“If you’re going to reduce the stockpile way down, then you’ve got to make sure that every bit of it works,” Kyl said.