Former Massachusetts Gov. Mitt Romney (R) urged senators on Tuesday against ratifying the new nuclear weapons treaty the president signed with Russia earlier this year.

Romney, a potential 2012 challenger to President Barack Obama, said that the New Strategic Arms Reduction Treaty (New-START) was Obama's "worst" foreign policy mistake to date.

"Whatever the reason for the treaty's failings, it must not be ratified," Romney wrote in an op-ed for The Washington Post. "The security of the United States is at stake."

The former 2008 Republican presidential candidate said the treaty, which seeks bilateral disarmament of nuclear weapons, cedes too much strategic and tactical advantage to Russia. Loopholes would allow Russia to keep many of its weapons while making it difficult for the U.S. to pursue its national security goals, Romney argued.

Obama signed New-START with Russian President Dmitry Medvedev in early April during a trip to Prague. The agreement was intended to replace an earlier iteration of the treaty, which expired in late 2009.

"By all indications, the Obama administration has been badly out-negotiated," Romney wrote of that agreement. "Perhaps the president's eagerness for global disarmament led his team to accede to Russia's demands, or perhaps it led to a document that was less than carefully drafted. "

Preliminary indications from senators in both parties had suggested that the treaty could face an uphill climb in the Senate, where 67 votes are needed for ratification. That's higher than the usually difficult threshold of 60 votes needed to break a filibuster.

But the treaty has steadily advanced through the Senate Foreign Relations Committee over the past month, where Sen. John Kerry (D-Mass.) and Sen. Richard Lugar (Ind.), the committee's ranking Republican, have announced a steady stream of bipartisan support from former diplomats, lawmakers and military leaders for the agreement.

Obama has said he'd like to see the treaty ratified by this November's elections.

Romney said that de-linking the U.S. missile defense system from the treaty was a critical element for the Senate in its consideration of the agreement.

"Then it must insist that any linkage between the treaty and our missile defense system be eliminated," he said. "In a world where nuclear weapons are proliferating, America's missile defense shield must not be compromised. As currently drafted, New START is a non-starter."