President Carter warned yesterday that a new global agreement designed to lower barriers to trade faces a tough time in Congress, and he urged the nation's governors to help him win its approval.

Carter said the set of agreements about to be reached by 98 nations now meeting in Geneva would "reduce existing protectionism and prevent the selfish protectionist tendencies that have always existed in every country, including our own, from prevailing in the future."

Addressing a winter meeting of the National Governors' Association here, he predicted, "This is not going to be an easy agreement to have ratified or approved by Congress.

"I hope all of you will study the details of those agreements and the benefits that we can derive from them," and "use your own influence... to encourage the Congress to approve these agreements once they have been reached."

The trade package, which would be the first modernization of the world's trading code in more than 30 years, is subject to U.S. enabling legislation. By law, Congress must vote the package up or down without amendment. Each house has 60 working days to act on the measure.

As if to underscore Carter's assessment of the difficulty that the package faces, Rep. Barber B. Conable Jr. (R-N.Y.), who was taking part in a panel discussion that the president joined, told Carter: "We've got to be convinced Americans can be benefited by increased export trade."

Carter praised the governors' association for creating an international trade committee last November, saying it has "tremendous, exciting potential."

He added, "The foreigner tag, which used to be a source of vituperation. Has now become a matter of an avenue for new friendships. If this committee had been formed back in 1936 or 1938, we might very well have avoided war with Japan.

"If there had been constant multiple avenues of commerce and trade and trade missions and governors' exchanges and the Congress working closely, we might very well have avoided the breakdown in relationships with Japan. Nobody knows that. But there is no doubt in my mind that we can alleviate tensions and search out new avenues not only for commercial benefit but new avenues forpeace..."

On domestic matters, the governors voiced concern over indications that Congress, now being pressed by the states to balance the federal budget, may wind up cutting them out of federal revenue-sharing funds this year.

Gov. Julian M. Carroll of Kentucky, association chairman, said such a move, proposed by Sen. Lloyd Bentsen (D-Tex.) and others, would be "very devastating."

Carroll told a news conference opening the governors' three-day session at the Hyatt Regency Hotel here that most states have already adopted their budgets for fiscal 1980 and are relying on general revenue-sharing money to balance their accounts.

The federal revenue-sharing program, which started in 1972 and is scheduled to expire next year unless renewed by Congress, now sends $6.7 billion to states and localities. The states' share is $2.28 billion.

Carroll said most governors are prepared to see the funds cut off in the fiscal 1981 budget because Carter campaigned in 1976 on a pledge to end the program for states when it comes up for renewal.

But later the association's executive committee passed a resolution making clear it wants the states left in the program both this year and in subsequent years.

Earlier, California Gov. Edmund G. (Jerry) Brown Jr said on the ABC-TV "Issues and Answers" program that "if Congress is prepared to get serious about balancing the budget, I'm prepared to give up state revenue-sharing." But he said Congress should look to military spending, the Health, Education and Welfare Department's budget and other federal programs for additional cuts.

Brown was asked about his state legislature's refusal last week to join 28 other states in asking Congress to call a convention to propose a constitutional amendment requiring a balanced budget.

"I don't consider it a defeat," Brown said, "I consider it a temporary setback in a long struggle to bring fiscal responsibility to the country."

At his news conference, Gov. Carroll said he thinks many governors favor a balanced budget amendment to the Constitution but are opposed to calling a convention. He said he personally favors such an amendment but feels that a convention "would be a very reckless way to amend the Constitution." Last August, the association adopted a resolution favoring a balanced federal budget but declined to approve one calling for an amendment.

A United Press International poll released yesterday showed that 26 governors oppose or lean against an amendment, 20 favor it, one declined to comment and three did not respond. The UPI poll also showed that 25 governors oppose an amendment that would put a cap on federal spending, 15 favor it, seven refused to comment and three did not respond.

Privately, several governors speculated about the possibility that Brown will challenge Carter for the Democratic presidential nomination next year. When asked about it on the television program, Brown said, "It's something I'm mulling over. Obviously, I'm concerned about the economy and the country."

Later he added, "It's only 1979. It's early. I have plenty of time to mull it over, and that's exactly what I intend to do -- a little more mulling."

Asked if Carter is in trouble politically, Brown said the president "is in the same position as many political leaders are in this country. There's a great lack of public confidence in our institutions. It affects governors as well as presidents and senators..."

Brown said there is a "discrepancy between the anti-inflation rhetoric coming out of both political parties coming out of both political parties over the last decade and the reality of rising inflation and the erosion of peoples' wages and pensions. That does put the president in some trouble. It places the general political establishment in trouble."