The Senate today voted down an effort to ban most U.S. underground nuclear testing, rejecting claims by supporters of the ban that it would help stem the atomic arms race. The chamber voted 57-39 to kill the proposed ban as it worked through a Pentagon budget bill. ``We must maintain unquestionable confidence in our nuclear deterrent and our experts tell us that this requires continued testing,'' said Sen. Strom Thurmond, R-N.C., a chief opponent. Sen. Mark Hatfield, R-Ore., the sponsor of the ban, said, ``no other issue addresses so clearly the runaway technology which is the lifeblood of the arms race.'' The House has approved an underground test ban in its version of the defense bill, meaning the issue will have to be resolved by a House-Senate conference committee that will resolve differences between the two measures. The House passed its defense bill Wednesday and the chairman of the House Armed Services Committee said the U.S. military is ``stronger and more capable'' now than when President Reagan took office, despite Reagan's recurring complaints about not getting enough money for the Pentagon. ``By almost any measure, our armed services are better than they were in 1981,'' said Rep. Les Aspin, D-Wis. ``The most dramatic increase is the quality of people, which is much, much better.'' Aspin commented shortly after the House voted 252-172 for a Pentagon budget, which will be the last defense budget of Reagan's eight-year presidency. House approval split along party lines, with majority Democrats supporting the bill and Republicans opposing it. Reagan didn't like it either, joining complaints by House Republicans that the bill unfairly restricts his arms control policies and Star Wars program. The House measure bans Star Wars tests that violate the so-called ``narrow'' interpretation of the 1972 Anti-Ballistic Missile treaty, and bars the purchase of any nuclear weapons that violate the numerical limits in the unratified 1979 SALT II nuclear arms treaty. The Senate, by a 51-45 margin, rejected an attempt Wednesday to add the SALT II restriction to its defense bill. Reagan opposes both restrictions and is also unhappy with the $3.5 billion approved by the House for the Strategic Defense Initiative, as Star Wars is formally known. The Senate voted Wednesday for $4.6 billion. Reagan wanted $4.8 billion for Star Wars next year, compared with the current SDI budget of $3.9 billion. Aspin said he couldn't understand the presidential complaints. ``I don't believe the president has any reason to be upset. I think it's a good bill. We gave the president a lot of what he wanted.'' Both the House and Senate bills authorize $299.5 billion worth of defense spending in the fiscal year starting Oct. 1, a slight reduction from the current $300 billion Pentagon budget. The measures would essentially freeze defense spending for a fourth straight year, following four large annual increases in Reagan's first term. No increases are likely in the future, Aspin predicted. ``I would guess that you're looking at perhaps a freeze, or maybe a decline of 1 or 2 percent, for the foreseeable future, which is the next three or four years, no matter who is elected president.'' Several factors will contribute to the reluctance to increase defense budgets, said Aspin. ``First, there are the worries about the federal budget deficit. Then second, you have the fact that the Soviets appear to the American public to be less threatening under Mikhail Gorbachev, so there's less a push because of fear. ``You also have the fact that there are other fears, like drugs,'' he said, noting that the House bill orders Reagan to use the U.S. military in the fight against illegal drug smuggling. ``In addition, there is a growing perception that national security depends on other things than simply military might, that it needs to include economic strength,'' he said. While the House measure restricts Reagan's arms control policies and Star Wars, it granted his proposals for a vast variety of weapons. That total includes 180 F-16 jet fighter planes, 84 F-A-18 jets, 42 F-15 jets, a dozen F-14 jets, three new Navy destroyers, two more attack submarines, one Trident missile-firing sub, the first of the new ``Seawolf'' class subs and 611 M-1 tanks. The Senate decision on Star Wars came as the chamber voted 50-48 against a deep reduction in the 5-year-old program. That vote defeated a proposal by Sen. Bennett Johnston, D-La., to reduce the SDI budget to about $3.8 billion and devote the extra $750 million to the space shuttle and a space station.