PRAGUE - The nuclear weapons cuts President Barack Obama and Russian President Dmitry Medvedev signed on Thursday would shrink the Cold War superpowers' arsenals to the lowest point since the frightening arms race of the 1960s. But they won't touch the "loose nukes" and suitcase bombs seen as the real menace in today's age of terrorism.

"This ceremony is a testament to the truth that old adversaries can forge new partnerships," Obama declared. "It is just one step on a longer journey."

The warheads covered by the treaty are lethal relics of the Cold War, and even with the planned reductions there will be enough firepower on each side to devastate the world many times over. And of more immediate concern are attempts by terrorist groups such as al-Qaida and nations such as Iran and North Korea to acquire or use nuclear weapons.

Obama and Medvedev showed solidarity for a spring showdown with Iran. And, beginning Monday, leaders of 47 countries will gather in Washington in an effort to prevent the spread of nuclear weapons, crack down on illicit nuclear trafficking and lock down vulnerable nuclear materials around the world.

Introduced Thursday with trumpet fanfare, the two grinning presidents sat at an ornate table in Prague's hilltop presidential castle and put their signatures to a landmark successor to the 1991 Strategic Arms Reduction Treaty. Nearly a year in the making, the "New START" signaled a bold opening in previously soured U.S.-Russia relations. If ratified by both nations' legislatures, it will shrink the limit of nuclear warheads to 1,550 each over seven years, down about a third from the current ceiling of 2,200.

Ratification in the U.S. Senate will hardly be automatic, requiring 67 votes in the 100-member chamber during a congressional election year when cooperation can be hard to come by.

Beyond that, urgent international nuclear tasks still face the two leaders.

For example, they are trying to forge agreement among themselves and four other nations - China, France, Britain, and Germany - on how to tackle Iran's continued defiance of United Nations demands that it cease enriching uranium. The West insists Tehran seeks to develop nuclear weapons; Tehran says it is after peaceful nuclear power.

At Obama's side, Medvedev made Russia's support for considering a fourth round of U.N. sanctions on Iran clearer than ever. "We cannot turn a blind eye to this," he said of Tehran's intransigence.

But that was not the main question heading into the leaders' talks, which ran overtime to about two hours. At issue, as representatives from the six partners prepare for what Obama called "ramped-up" discussions in New York, is how weak any new sanctions regime would need to be to get Moscow on board - not to mention China, an even more stubborn holdout.

Medvedev said sanctions should be "smart" - designed to change behavior, not to bring down the hardline Iranian government or impose hardship on Iran's people. The Russian leader said he had outlined for Obama "our limits for such sanctions," and Obama Russia expert Mike McFaul said those discussions got very specific.

"In all negotiations, people talk about their red lines and their bottom lines and we negotiate," McFaul said. White House officials would not reveal details of the private conversation, concerned that it could threaten progress. But Russia's deputy foreign minister, Sergei Ryabkov, said that a total embargo on refined petroleum products into Iran, which depends heavily on such imports, was out of the question for Moscow.

There is talk of hitting refined petroleum product deliveries some other way, but sanctions on Iran's energy sector may be jettisoned altogether as too tough for Russia or China.

Obama repeated his flat declaration that "strong tough sanctions" will be agreed to this spring. He said "we will not tolerate" any actions by Iran that risk a new arms race in the Middle East or threaten the security of the international community.

The president faces another key test in that drive when he meets Chinese President Hu Jintao in Washington on the sidelines of Monday's 47-nation summit.

Even as the U.S.-Russia deal was signed in Prague, the White House was deeply engaged in the uncertain Senate ratification fight in Washington.

With Obama needing to cajole at least eight Republicans into supporting the treaty to win the required 67 votes, Brian McKeon, a senior foreign policy adviser to Vice President Joe Biden, was named to head the effort. Administration negotiators also began fanning out to lobby senators, in person and via videoconference from Prague.

Fearing potential trouble, Medvedev said Russian lawmakers will synchronize their moves to ratify the deal on the U.S. timeframe.

Both leaders expressed optimism, and Obama emphasized the history of Senate bipartisanship on arms control matters. But that could be wishful thinking this year.

The GOP could well see an irresistible opening to criticize the broader security policies of Obama and his Democratic allies. Even if Republicans don't reject the treaty, they could seek to postpone its ratification to deny Obama a victory ahead of the November midterm elections.

One potential GOP backer, Richard Lugar of Indiana, a moderate Republican steeped in nonproliferation issues and the top GOP lawmaker on the Senate Foreign Relations Committee, has been curiously quiet.

But Republicans are expected to eventually swing behind the treaty if Obama can promise it won't undercut the nation's ability to set up missile defenses to protect against an attack from Iran or North Korea. Senate Republican leader Mitch McConnell said the GOP also wants assurances that the agreement will preserve the nuclear triad, a reference to the three tiers of the nation's nuclear defense.

Senate Majority Leader Harry Reid, D-Nev., issued a statement welcoming the treaty and warning Republicans not to "play politics with something as important as this to our national security." He said he was confident the agreement would be ratified.

Obama said the U.S. wants to get started on more arms-control negotiations with Russia, seeking larger cuts and ones that target short-range nuclear weapons as well as those held in reserve and in storage. None of those are affected by New START.

There are many reasons that any follow-on arms reductions will be much more difficult to achieve, including the missile defense dispute, the Russians' larger reliance on nuclear weapons in their overall security strategy and the need to draw in third powers.

Asked about the prospects, Konstantin Kosachev, the Kremlin-connected chief of the foreign affairs committee in the lower house of Russia's parliament, said it is a nonstarter until the U.S. withdraws its tactical nuclear weapons from five countries in Europe.