WASHINGTON — Democrats in Congress are building a legislative roadblock to the Bush administration’s plan to place elements of a missile defense system in Poland and the Czech Republic.

The Senate is expected to join the House next week in reducing funds in the fiscal 2008 Defense Authorization Bill for construction of 10 interceptor missile sites in Poland and for deployment of an X-band radar in the Czech Republic. The House, in passing its version of the legislation last month, cut $40 million from the bill, which would have funded preparation of the Polish sites next year. The move prevents the White House from proceeding unless President Bush vetoes the measure.

The Senate Armed Services Committee, justifying its decision to delay funding for the European antimissile sites, emphasized not only foreign policy concerns but also technical issues surrounding the interceptor missiles.

As illustrated by President Putin’s statements last weekend during his visit with Mr. Bush in Maine, Russia remains opposed to the American government’s deployment plans. In its report on the authorization bill, the Senate committee cited Moscow’s opposition and said any funding should await completion of talks between the two governments.

The panel also noted that moving ahead without NATO participation in a system that is in part intended to protect Europe from Iranian missiles would cost America an estimated $4 billion through fiscal 2013. “NATO, which has not yet decided to pursue missile defense of its territory, has not endorsed or rejected the proposed deployment,” the panel’s report added.

Although preliminary talks have begun with Polish and Czech leaders, the committee said, “These negotiations may not be concluded before the end of this year, and then would have to be ratified by the parliaments in each nation.” At the same time, residents of the areas where American facilities would be located have voiced overwhelming objections in local referendums.

Tomas Klvana, the Czech special envoy for missile defense, recently told reporters in Washington that some of the opposition has been generated by anti-Americanism and by concerns about foreign soldiers in the country. “We have some catching up to do,” Mr. Klvana said to the group, brought together by the German Marshall Fund. “We have given a free ride to the people opposed.”

The U.S. Missile Defense Agency has told the Senate panel that construction and deployment could not begin until the two countries ratify agreements with America, estimating that such actions “would not take place before 2009,” according to the report. Polish officials, hearing Russian objections to the U.S. system so near their border, have also raised questions about the additional U.S. military support that would come with placement of the interceptors on their soil.

The Senate panel also noted that the interceptor to be placed in Poland “has not yet been developed or tested, and is not currently planned to be flight-tested until 2010.” Therefore, the committee concluded, “it could be several years before it is known if the interceptor will work in an operationally effective manner.”

The committee also raised in its report the timing of a potential Iranian threat.

Air Force Lieutenant General Henry Obering III, director of the Missile Defense Agency, recently testified before the panel that U.S. intelligence misjudged North Korea’s ability to launch a long-range missile in 1998. “Right now, the experts are saying that Iran will not have an ICBM until the 2010–2015 time frame,” General Obering said. “But it’s going to take us at least that long, until 2011 or 2012, to get a first capability in the ground.”

Pushing to move ahead rapidly with a missile defense system, he added: “What we’re trying to do is stay ahead of what we believe to be an emerging threat, because we can’t wait until they actually demonstrate it and then say, ‘Now let’s go find a way to counter it.’ “

The Senate committee, according to its report, apparently did not embrace that argument . “There is uncertainty about whether Iran will have such long-range missiles, or nuclear warheads that could work on such missiles, by 2015,” it said.