House Democrats gained surprising Republican support yesterday to beat back a series of amendments designed to weaken a trade bill the Reagan administration has attacked as highly protectionist.

As many as half the House Republicans appeared to be voting regional economic interests over the party line to support a broad-based bill crafted by the Democratic leadership.

A final vote is scheduled for today on the bill, which Democrats hope will make the record trade deficits during the five years of the Reagan presidency a major November campaign issue. While its passage by the Democrat-controlled House is assured, the White House is trying to cut into the number of Republican defections in order to paint the bill as a partisan measure.

U.S. Trade Representative Clayton Yeutter said the administration wasn't twisting arms to keep the 182 House Republicans in line. "They can make their own decision on how they want to vote," he told reporters.

Farm state Republicans, concerned that some of the bill's actions against unfair trade practices would spawn retaliatory moves that would hurt U.S. agriculture sales, supported administration efforts to gut the measure.

But 98 Republicans -- largely from Rust Belt and timber-producing states -- joined with 240 Democrats to defeat an amendment that would strip the bill of provisions strengthening U.S. laws against other countries' unfair trade practices. Republicans hoped to lose no more than 60 votes -- one-third of the House GOP members.

The amendment, sponsored by Rep. William Frenzel (R-Minn.), would remove a provision that defines cut-rate sales of natural resources such as timber or oil as a subsidy and an unfair trade practice. That part of the Democratic bill is aimed at Mexican and Saudi Arabian oil sales and Canadian timber.

The amendment offered by Frenzel, a longtime supporter of free trade who was the main administration spokesman on the House floor, was defeated by a 338-to-79 vote, with three Democrats crossing party lines.

Earlier, the House defeated another Frenzel amendment by a 306-to-107 vote with 65 GOP defections. That amendment would restore presidential discretion, removed in the bill, to deny trade relief to industries hurt by imports and would remove sections that promote worker assistance and industrial restructuring.

The House also defeated an attempt by Rep. Philip M. Crane (R-Ill.) to kill a part of the bill that the Reagan administration found most objectionable. This provision, sponsored by Rep. Richard Gephardt (D-Mo.), would force reductions in the "excessive" trade surpluses of Japan, Taiwan and West Germany if they refuse to end their pattern of unfair trade practices.

The Crane amendment was defeated, 276 to 137, with 44 Republican crossovers.

The House beat back a Republican attempt, supported by Defense Secretary Caspar W. Weinberger, to gut parts of the bill that ease export controls by reducing by 40 percent the number of items on the list of militarily significant products and technologies that should be kept from the Soviets.

Weinberger called that part of the bill a gift to Moscow and terrorism. But Rep. Don Bonker (D-Wash.), holding a small calculator he said was on the list of controlled products, argued that reducing the list of critical items would make U.S. export control efforts more effective while promoting American high technology sales. Chairman Dante Fascell (D-Fla.) insisted that the provision would not hurt national security.

The amendment, offered by Rep. Toby Roth (R-Wis.), was defeated 236 to 182. There were 42 Republican votes against it despite strong Defense Department lobbying for the Roth amendment.

In the closest vote of the day, the House defeated, 221 to 188, a Republican effort to strip the bill of sections dealing with monetary policy and Third World debt. Rep. Stan Lundine (D-N.Y.), main supporter of those parts of the bill, said the administration opposed them even though they provide a congressional underpinning to a program of Treasury Secretary James A. Baker III to stabilize international currency fluctuations and to relieve the debt crises in underdeveloped countries.

Republicans and Democrats joined to pass a nonbinding resolution, 408 to 5, demanding Japan give full access to American semiconductor exports and calling for a determination by President Reagan as to whether Japan has used unfair trade practices to become a world power in semiconductor sales.