Once the Senate Republicans carry out their threat to block and kill the new Strategic Arms Reduction Treaty (New START) this year, the United States' ability to conduct foreign policy effectively throughout the world will be significantly weakened.

The treaty's failure has the obvious consequences of harming relations with Russia and potentially undermining cooperation on Iran, Afghanistan, and securing nuclear materials, and it will make it harder for all foreign governments to take political risks in negotiating future agreements with the United States. In addition to raising doubts about President Barack Obama's ability to win support for accords he has signed, the treaty's fate will show the world that every administration initiative, no matter what it is, will be subjected to constant opposition for narrow political ends. Contrary to most expectations, the recent midterm election results have not just had some impact on U.S. foreign policy, but also are immediately having an outsized, disruptive effect that seems likely to increase during the next two years.

Many foreign governments may decide that it is better to wait until after the next election before entering into serious negotiations with America over anything. Russian President Dmitry Medvedev expended considerable political capital on negotiating the arms reduction treaty, and its defeat will be an embarrassment for him. Other foreign leaders will not want to expose themselves to the same risk, and will become much more reluctant to offer politically sensitive concessions. The results will satisfy neither hawkish interventionists nor conservative realists, and they are bound to horrify liberal internationalists. At a time when international summits are expanding to accommodate more major and rising powers and America cannot readily count on the support of other governments, the United States needs to have even greater credibility abroad. The treaty's opponents are making sure that America will have considerably less credibility than it already does, which worsens the chances of U.S.-directed collective action on any number of issues from proliferation to climate change to conflict resolution.

For their part, unilateralists will also have little reason to celebrate. For all of the nonsensical fretting hawkish administration critics have done about the "post-American" Obama, they are unwittingly hastening the emergence of the "post-American" multi-polar order that they loathe. American hawks want the United States to remain a European power and to exercise leadership through NATO, but treaty opponents are sabotaging an agreement that NATO and its member governments strongly endorse as important for their own security. Administration critics have been captivated by the false notion that Obama has been abandoning U.S. allies, but it is treaty opponents who will be leaving them in the lurch.

The administration's ability to isolate Iran diplomatically and economically will be undermined as Russian cooperation melts away, but according to the perverse logic of U.S.-Iran relations this will make confrontation more rather than less likely. As international support for Iran sanctions weakens, the more support there will be at home for harsher U.S. sanctions, and there will be a steady escalation of tensions that could break out into direct conflict. Even if one believes that the administration’s diplomatic efforts to isolate Iran have been badly misguided, as I do, the easing of international pressure on Iran makes it more difficult for the administration to consider a genuine policy of engagement.

To a large degree, the president has been operating on the assumption that the bipartisan consensus on foreign policy that prevailed after the end of the Cold War is stronger than the desire for partisan and ideological advantage. That no longer seems to be true. Despite myths to the contrary, American politics has never stopped at "the water's edge." U.S. foreign policy has long been subject to the pressures of partisan competition and opportunism, but the sharp divergence of foreign policy priorities of the leadership of the two parties and the willingness to exploit major foreign policy issues for purely political goals are relatively recent developments. This may be an unavoidable byproduct of the divisions over the Iraq war and the relative decline in American power, but it is something that threatens to introduce a new degree of volatility and instability into U.S. relations with other nations. Intense, reflexive opposition to President George W. Bush and President Obama may be the cause, but the United States will have to live with the effects of this instability long after they have been out of office.