President Barack Obama appealed Saturday to Republicans in the U.S. Senate to stop blocking a nuclear arms pact with Moscow, saying failure to soon ratify it could jeopardize improving relations with Russia and send a mixed signal to Iran about the strength of the international front against its nuclear program.

He blamed the supercharged partisan climate in Washington for the delay and said inaction on the pact would leave "a partner hanging" at a time of better cooperation among the United States, its NATO partners and Russia.

Obama said European allies at the NATO summit told him that the stalled treaty is critical to U.S.-European security. He talked with reporters after the 28-nation alliance met with Russian President Dmitry Medvedev.

Obama used his weekly radio and Internet address to press his case, noting that Russia had voted with the U.S. and other allies to impose the latest round of U.N. penalties against Iran over its nuclear program. Russia is a partner with Iran in a civilian nuclear power project and generally has been less concerned than the U.S. that Iran may be hiding a bomb program.

Obama suggested Republican senators standing in the way of the pact with Russia were abandoning Ronald Reagan's lesson of nuclear diplomacy: "Trust but verify."

The treaty would limit each country's stockpile of nuclear warheads to 1,550, down from the current level of 2,200, bringing the arsenals to a level last seen in the 1950s. It would replace the 1991 Strategic Arms Reduction Treaty, or START I, which expired last December.