WASHINGTON — As the Bush administration speeds ahead with plans to construct a missile defense system in Eastern Europe, some Democrats in Congress want to put on the brakes, saying it has not been adequately tested.

Secretary of State Condoleezza Rice was scheduled to be in Warsaw today to sign an agreement on the missiles with Poland, which agreed to the basing of 10 interceptors last week, after the Russian attacks on Georgia. Justified as a defense against a missile attack on Europe by a rogue nation like Iran, the installation has provoked outrage from Russia.

Even before the agreement was reached, the Bush administration had proposed spending $712 million in the coming fiscal year to start digging silos in Poland; installing a related radar system in the Czech Republic, another former Soviet satellite that is now a NATO member; and buying initial parts for the first interceptor missiles.

But Democrats are now questioning all that spending as premature.

"Go ahead and move on with research and development," said Rep. Ellen O. Tauscher, D-Calif., who is chairwoman of the House subcommittee that oversees the missile defense program. "But as far as putting holes in the ground in Poland, we are saying no."

The conflict between Georgia and Russia has complicated the debate.

While the Bush administration has long argued the missile defense system is not motivated by any Russian threat, Republicans in Congress intend to use the recent events to push for an immediate start of construction.

"It is going to be easier to make our case on Capitol Hill now," said Rep. Trent Franks, R-Ariz., a proponent of the program, "as this has reminded Poland and some of the other nations formerly dominated by the Soviet Union that the coercive Russia mind-set of militarily threatening your neighbors has not completely disappeared."

The two presidential candidates have also taken sides, with Sen. John McCain supporting quick construction while Sen. Barack Obama is urging caution, saying the system is unproved.

As designed, the European system would be the latest version of a long-range missile system that is already partially installed in Alaska and California.

It would be essentially useless against a Russian attack because the interceptor missiles that are based in California and Alaska, or that would be based in Europe, are so few that they could easily be overwhelmed by the Russian missile arsenal.

But if North Korea sent one or two missiles toward the United States, or Iran sent a couple toward Europe, the American system is meant to knock them down.

During the development of the system installed in the United States, which is now considered operational, the missiles went through a series of tests, and the Defense Department claimed successful intercepts in six of the nine tests conducted since 2001.

A promotional video released by the main contractor, the Boeing Corp., shows military personnel celebrating with high-fives after destroying a missile sent by a fictional enemy.

But the Pentagon's own test and evaluation office says the technology, particularly in the version planned for Europe, remains unproven.

The Europe-based interceptors would have less time to knock out the approaching threat, given the shorter distance between Iran and Europe than between North Korea and the United States.

The system in Europe would also rely on an untested two-stage rocket, instead of the three-stage rockets now used on the 24 interceptor missiles in California and Alaska.

Pulling it all together in Europe, the Pentagon's testing office reported last year, "will be a significant challenge."

More engineering work was needed before tests could prove the system's effectiveness, the office said.

Lt. Gen. Henry A. Obering III, director of the Missile Defense Agency, said the system, called the Ground-Based Midcourse Defense System, has proved through a series of continuing tests to be reliable, and that the changes being proposed were not radical.

"Is this a perfect system? Absolutely not," he said. "Is it embryonic? No, we are well beyond that."

This spring, the House Armed Services Committee voted to withhold authorization for most of the requested funds for the initial construction in Poland, and proposed language that would bar spending to build the system, until the secretary of defense certified that it was reliable.

Obering said he agreed that the missiles should not be deployed in Europe until the testing was complete, which he said was likely to be in 2010. But construction should proceed, he said.

"We can't wait until the Iranians launch a long-range missile and then start worrying about building out the site," he said. "If you do that, you are way behind the curve."

The conflict between Georgia and Russia has helped turn public opinion in Poland in favor of the agreement with the United States, according to a poll released this week by Rzeczpospolita, a leading newspaper there.

The Bush administration, as part of the package, agreed to place a battery of Patriot missiles — shorter-range defensive interceptors that could, at least in theory, be used against a limited Russian conventional attack.