Barack Obama faces Senate struggle to ratify 'crucial' arms control treaty US president closes Nato summit with appeal to approve New Start treaty struck with Moscow

President Dmitry Medvedev: 'I hope US legislators demonstrate a responsible attitude.' Photograph: Jason Reed/Reuters

President Barack Obama made plain last night that he was mounting a strong push to get a crucial new arms control treaty with Moscow ratified. Failure, warned President Dmitry Medvedev of Russia, could thwart hopes of a new era in co-operation with the west.

At the end of a Nato summit in Lisbon, attended by a Russian head of state for the first time since tensions soared after Moscow invaded Georgia in August 2008, Obama delivered a passionate appeal for the US Senate to ratify the New Start treaty agreed by Washington and Moscow in April.

"This is an issue of critical national security interest," he said, warning that his policy of "resetting" relations with Russia could hinge on the outcome in the Senate, which needs to pass the treaty by a two-thirds majority to complete ratification. "It would be a mistake to fall back into mistrust as a result of our failure to ratify."

The president said the treaty slashing both sides' strategic nuclear missiles was backed by America's allies, the Pentagon, national security advisers and defence secretaries of both main parties. The treaty had gone through 18 congressional hearings and 1,000 questions.

"It's time for us to go ahead and get it done."

He lamented that because of the recent mid-term elections that left him weakened and the Republicans buoyant, Washington had become "a partisan place. We have to rise above this partisanship."Obama and Medvedev signed the treaty in Prague following the expiry of its predecessor last year, meaning that neither side is able to keep an eye on the other's disarmament.

Medvedev said that yesterday's summit proved that Russia and the west had travelled a long way since "the delusions of the 1990s" and the tensions triggered by the Georgia conflict. But he said failure by Washington to get the treaty adopted could bring an unravelling of developing ties on issues from Iran to Afghanistan, missile defence and counter-terrorism. "I hope US legislators demonstrate a responsible attitude," he added.

Analysts say that Republican gains in the recent Senate elections mean the treaty is doomed unless Obama gets it ratified before the new senators take their seats in January.The republican whip, Senator Jon Kyl, is the focus of the presidential drive. "Senator Kyl has never said that he doesn't want Start ratified," said Obama. "I take him at his word."