The US lame-duck Congress on Monday reassembles for its final session. The most important thing it must do is ratify the Strategic Arms Reduction Treaty (Start), signed in Prague last April by Presidents Obama and Medvedev, cutting the US and Russian strategic nuclear arsenals. If not now, it may be never – and the consequences could be dire.

But why the fuss, it will be asked? The Cold War is long since over, the two countries are no longer locked in an ideological and military battle for global supremacy. And the new treaty doesn't even impose very dramatic reductions; can't it wait? The answer is, no.

Right now, the required two-thirds majority of 67 votes is just about there in the outgoing Senate, where Democrats have a 59-41 advantage. Come January, when a Tea Party-driven Republican Party takes control of the House and adds six seats to its strength in the Senate, that will no longer be the case. Conservatives will be in the ascendancy, making the familiar, overblown argument that the treaty sells US national security down the river.

Ratification, they and indeed some liberals here will add, would also signify tacit US acceptance of the growing authoritarianism and general lawlessness of the Russian government (as in the brutal and unpunished attacks against journalists whose work displeases those in power, and the farcical retrial – or rather show trial – of the former oligarch Mikhail Khordokovsky).

Right on cue, that old neo-con warhorse John Bolton popped up in The New York Times yesterday, claiming that the new treaty was a sell-out on missile defence, and would gravely weaken the American "nuclear umbrella" that underpins international security. Congress, he argued, should have nothing to do with it. But such arguments do not hold water.

First and foremost, for the US to shelve this treaty would send – not for the first time – a message about the hypocrisy of Washington's efforts to halt the spread of nuclear weapons, above all, of course, to Iran. The fragile edifice of non-proliferation rests on a bargain between the nuclear and non-nuclear powers. The former will not try to acquire nuclear weapons if the former reduce, and ultimately eliminate, their own arsenals. If Start is not ratified, the US will yet again stand guilty of telling the world: "Do as I say, not as I do".

As for pulling the rug from under US security, the treaty stipulates that after seven years neither side will be allowed to deploy more than 1,550 strategic warheads, or 700 launchers. In fact, the actual changes, thanks to past reductions and abstruse counting rules, will be relatively small. But the treaty reinstates an important inspection regime, and improves the climate for bigger cuts in the future.

Senate inaction (or, worse still, rejection of the treaty) could also have an impact on Russian internal politics – and one that would slow down progress on what human rights advocates here and everywhere desire: the transformation of Russia into a modern, law-governed state.

Right now, Moscow is consumed by a single question: who will be the establishment candidate in the 2012 presidential elections? Will it be the conservative former president Vladimir Putin, or his hand-picked successor, Dmitri Medvedev? It may of course already be a done deal, and Putin, ever the organ-grinder to Medvedev's monkey, has stitched up victory in advance.

But the current fierce jockeying within the Kremlin suggests otherwise. If so, then the 2012 election is shaping up as another instalment of the age-old struggle for Russia between Westernisers and Slavophiles.

In that case, the US and its allies are surely rooting for Medvedev who, however imperfectly, appears to represent modernising and Westernising forces within Russia, and who offers the best hope of improvement on the human rights front. Medvedev also has been the prime mover on the Russian side in efforts to "re-set" relations between Washington and Moscow, after the deep chill of the later Bush years. He happens, too, to be one of few foreign leaders with whom Obama has established genuinely close personal ties. Not to ratify Start would merely play into the hands of the Putin camp, and conservative nationalists who still smart at defeat in the Cold War and see a plot in every foreigner. As ever, they claim, the US wants to impose its will on Russia, and Congressional inaction on the treaty will be presented as further proof of that. The worst thing the US could do now is give a helping hand to an old guard seeking to turn the clock back. If the lame-duck Congress lets Start rot, it will do precisely that.

Obama won't find salvation in Asia

It's an old saw of US politics that when the going gets rough at home, a president heads for foreign parts. Richard Nixon sought refuge from Watergate in Soviet summitry, Bill Clinton went peacemaking in Ireland amid the Monica Lewinsky scandal. And now there is Barack Obama's trip to Asia, in the aftermath of his crushing midterm election setback.

The habit makes perfect sense. So often we forget that for all the might of his office, an American president's domestic power is limited – certainly in comparison to his French counterpart or a British prime minister. Foreign policy is where he has a pretty free hand.

But it's not exactly plain sailing for Obama abroad, either. The hopes inspired by his 2008 election, and the premature award of the Nobel Peace Prize a year later, have faded. He's been well received in India, and in Indonesia where he spent four years of his childhood. But in Seoul at the G20 he is walking into a currency war in which the US is widely perceived as villain. Maybe he should have stayed at home after all.

r.cornwell@independent.co.uk