The Republican Party took another step toward control of Congress today when the Democratic-dominated Texas legislature approved and sent to Gov. William P. Clements a congressional redistricting plan that could give the GOP an additional four seats in 1982.

The action, the GOP's biggest redistricting victory to date, exposed the disarray of the state Democratic Party. It also opened the way for a bitterly contested series of primary and general elections in 1982 that could mean a permanent political realignment in this longtime Democratic stronghold.

The victory was a personal triumph for Republican Clements, who won with the same combination of strict party discipline and conservative Democratic defections that pushed President Reagan's economic program through Congress earlier this year.

The new plan must be approved by the Justice Department under the Voting Rights Act, and a number of groups are considering challenges to it on grounds that it dilutes the Mexican-American vote. Court challenges also are likely, meaning it may be months before a final plan is ready. But because of the state's demographics, a map more favorable to Mexican-Americans could give the Republicans even greater gains.

"I think the House and the Senate and the leadership of both houses are due great credit for responding properly to the wishes of Texans," Clements said in a statement today. "Texas is a conservative state and the bill reflects what is right for Texas."

Clements had the active support of both Democratic House Speaker Billy Clayton, who authored the plan that was finally adopted, and Democratic Lt. Gov. William Hobby, who provided a key vote in the Senate a few weeks ago that broke the Democratic resistance there.

Today, eight Senate Democrats joined the eight Republicans in the 16-to-15 vote that guaranteed approval of the new map. Earlier in the House, 37 Democrats defected and joined with the 38 Republicans to provide a five-vote margin on the key vote there.

The Texas congressional delegation currently has 19 Democrats and five Republicans. Because of population increases, the state will receive three more seats in 1982. Democrats said today that the congressional plan approved by the legislature could result in the election of 18 Democrats and nine Republicans.

"The thing I resent most is that they let the governor do with a pencil on a map what the Republicans couldn't do at the polls," said Democratic State Chairman Bob Slagle. "You ought to beat somebody electorally, not with a pencil."

The new plan puts Dallas Democrat Jim Mattox into a redrawn Republican district, and Mattox today said he is not certain he will run for reelection there. It also threatens the reelection of Martin Frost, another Dallas Democrat whose new district is made up of more than 60 percent minority voters. Clements and a black coalition in Dallas successfully joined forces to win this new minority district in a fight that split the state's black leadership.

The new map also gives the Republicans a new district between Dallas and Fort Worth.

In addition, the plan threatens Democratic Reps. Ralph Hall, one of the congressional boll weevils who has supported President Reagan on nearly every key vote in Congress this year, and Bill Patman, a freshman Democrat who tangled with the chairman of the state House redistricting committee, Tim Von Dohlen. Von Dohlen is believed to have his eyes on Patman's seat.

Why Clayton sided with Clements is not clear, although the speaker is one of the most conservative Democrats in the legislature. There is some speculation that Clayton fought for a conservative plan in return for the governor's support for a water resources trust fund bill that may provide him with a platform for a statewide race for land commissioner in 1982. There also has been talk that Clayton might switch parties in 1982, but he has denied it. He argues that his plan would give Democrats 20 seats, Republicans six, with one a tossup.

Final action on the redistricting bill came a day before the end of a special session called because the legislature failed to write a redistricting plan during its regular biennial session that ended June 1.

When the special session convened in July, Democrats hoped to win quick approval of a plan protecting all incumbents that was drawn in Washington by Rep. Frost with the support of House Majority Leader James C. Wright of Texas. But veto threats from Clements, plus a well-financed lobbying campaign by a group called Texans for a Conservative Congress, whose membership included many of the wealthiest businessmen in the state, forced the state Senate to reject that plan in favor of one sponsored by conservative Democrats that opponents said would have elected 20 Democrats and seven Republicans.

Democrat Clayton then produced his own plan that was even more favorable to the Republicans, and after much legislative maneuvering, won approval for it early Sunday morning. Today, the Senate concurred with the House version, even though Clements was originally satisfied with the less generous Senate plan.