President Carter's proposed "real wage insurance" tax credit won a strong endorsement yesterday from the United Auto Workers Union, but there still were no firm indications on whether the plan would be approved by the House Ways and Means Committee.

In testimony before the panel, UAW vice president Patrick Greathouse urged the lawmakers to pass "framework legislation" approving the plan in principle before this year's major wage-bargaining begins, and leave the details to the White House to work out later.

Greathouse termed the tax credit "the linchpin" of the administration's new anti-inflation effort, and warned that if the proposal is not passed his union will not commit itself to following President Carter's 7 percent wage guideline this year.

The UAW's endorsement was accepted noncommitally by the panel. Ways and Means Chairman Al Ullman (D-Ore.) rejected the nation of enacting "framework" legislation without also prescribing the details of the plan, but made no predictions on whether the committee would aprove the plan Carter has proposed.

Members of the panel generally have been unenthusiastic about the tax-credit proposal, but the committee's Democratic majority is said to be willing to approve the measure if Ullman and other leaders push for it. A key decision could come next week after the panel completes its hearings.

Meanwhile, the administration received a setback as the bargaining policy committee of the United Rubber Workers union reaffirmed earlier assertions that it would not be bound by the administration's voluntary 7 percent wage guideline in its coming contract talks with companies.

Instead, the panel, which includes rank-and-file delegates as well as local union presidents, indicated it may seek larger cost of living increases and more job security as well as big wage hikes. However, URW president Peter Bommarito said he hopes to avoid another prolonged strike.

The UAW's endorsement of the real wage insurance proposal was tempered in part by criticism of some of the measure's key provisions, including Carter's plan to deny the tax credit to some low-wage workers. However, Greathouse told the committee the union still backs the overall bill.

At the same time, Lucy W. Peterson, an official of the National Small Business Association, opposed the real wage insurance plan was unlikely to be effective. Most business groups have criticized the proposal in testimony this past week.

The real wage insurance plan is designed to protect workers who agree to follow Carter's guideline by offering them a tax credit amounting to 1 percent of their first $20,000 in wages for each percentage-point that inflation exceeds 7 percent.