For the first time in 15 years, U.S. officials have lost their ability to inspect Russian long-range nuclear bases, where they had become accustomed to peering into missile silos, counting warheads and whipping out tape measures to size up rockets.

The inspections had occurred every few weeks under the Strategic Arms Reduction Treaty. But when START expired in December, the checks stopped.

Meanwhile, in an obscure, fluorescent-lighted State Department office staffed round-the-clock, a stream of messages from Russia about routine movements of its nuclear missiles and bombers has slowed to a trickle.

The Obama administration hopes the inspections and messages will soon resume under the New START agreement, which was signed by the two countries in April. But the pact is on hold in the Senate. If it faces long delays, or is voted down, the U.S. government will lose critical insight into Russia's nuclear forces, officials say.

"The problem of the breakdown of our verification, which lapsed December 5, is very serious and impacts our national security," Sen. Richard G. Lugar (R-Ind.), one of the chamber's top nuclear experts, said in a recent hearing.

In months of debate over New START, there has been little focus on the implications of the lapse in nuclear checks. Instead, hearings have centered on such issues as whether the pact would inhibit U.S. missile defense.

"I thought we were just going to continue doing business as usual" as the replacement treaty was debated, Sen. Jon Kyl (R-Ariz.) said when a reporter noted the inspection cutoff.

The Obama administration has emphasized that New START will require the United States and Russia to reduce their nuclear arsenals. But many experts say the verification measures matter even more.

That's not because they think a nuclear attack is imminent. But even two decades after the end of the Cold War, Russia has about 2,500 deployed nukes capable of hitting the United States. U.S. officials like to keep an eye on them.

"Without the [new] treaty and its verification measures, the United States would have much less insight into Russian strategic forces, thereby requiring our military to plan based on worst-case assumptions," Jim Miller, a senior nuclear policy official in the Pentagon, testified last month. "This would be an expensive and potentially destabilizing approach."

Kyl and other Republicans say that before voting on a pact that reduces the nation's stockpiles, they want to ensure there is enough money to modernize the nuclear complex. They say they should not rush the treaty because the monitoring measures have expired.

"It's not an argument for voting before you know all the facts," said Senate Minority Leader Mitch McConnell (R-Ky.). For the Cold Warriors who plodded through arms-control talks back in the 1980s, getting inspectors onto the other guy's bases was a major breakthrough.

"It was the holy grail to get on-site inspections, boots on the ground in the Soviet Union," said Franklin Miller, who worked in arms control for more than two decades, ending up as special assistant to President George W. Bush. Even without those inspections, the U.S. and Russian governments can still check on each other's forces by using reconnaissance satellites and radar. But those methods are not perfect.

For example, a satellite cannot peer into a Russian underground silo and see whether the missile inside is carrying one nuclear bomb or 10, officials say.

"One of our dirty little secrets is, when the [Berlin] Wall went down, the United States reoriented a lot of intelligence capacity away from the Soviet Union and Russia. To some fair degree . . . the IC intelligence community] was relying on U.S. inspectors to be on the ground," Miller said.

The "boots on the ground" include people such as Phil Smith, a former Air Force crew chief for nuclear-tipped missiles. He has made about 20 inspection visits to Russian nuclear facilities.

"We have 15 years of experience under START, understanding where everything is. We've been through these sites multiple times," he said in an interview.

The U.S. teams typically arrive at Russian bases with only about a day's notice. Many of the inspectors' methods are surprisingly low-tech: They stretch tape measures along missiles and poke flashlights into trailers. The inspections allow each side to count nuclear weapons on a sampling of missiles, bombers or submarine launch tubes and look around one another's maintenance facilities and test ranges.

"If something is atypical . . . I will not be bashful about saying, 'Okay, we need to take a closer look at this one.' That's the kind of dynamic you have on the ground that you wouldn't have with a satellite," Smith said. Inspectors check what they see against a database compiled by both sides with the numbers, characteristics and locations of their long-range nuclear weapons.

Until December, both sides updated that database constantly. Russia sent about 1,500 notifications a year to a special computer at the State Department's Nuclear Risk Reduction Center, where a "ding-dong" would signal an incoming message. ("It sounds like Avon calling," explained one technician.)

The messages, which the center distributed to U.S. security agencies, included information on upcoming inspections, the destruction of nuclear launchers and movement of nuclear-capable missiles and bombers.

"Now we don't get any of that information. We have less and less visibility into their status of forces," said Ned Williams, the director of the center. (Notifications of missile test launches have continued, to ensure that neither side mistakenly thinks a nuclear attack is underway.)

Few experts dispute the value of having inspections. But some critics have argued that New START is not as good as its predecessor.

The Obama administration "agreed to gut the monitoring and verification measures and limitations necessary to render it effectively verifiable," said Paula DeSutter, the assistant secretary of state for verification in the George W. Bush administration.

For example, she said, the Obama administration acquiesced to a Russian demand to exchange less telemetry -- the flight data from ballistic missile tests. That information helps U.S. officials understand the number of warheads the Russians will load onto their missiles. Under New START, the Russians are required to provide the data from only five tests, instead of all 10 or 12 they do annually.

U.S. officials say the change is not significant because, under the new treaty, they will be counting the number of warheads on missiles and not using estimates, as was the case before. They contend that the new treaty will help each side get a more accurate count by assigning an ID number to each warhead and launcher.

Although U.S. nuclear inspectors are not traveling to Russia these days, they are busy training, sometimes with mock "Russian" inspectors. The idea, Smith said, is "to make sure when we're called upon to do this, we're ready to go."