President Carter's smoothly running campaign to "reform" the federal bureaucracy suffered a major setback here yesterday. Delegates to the convention of the American Federation of Government Employes - biggest and most powerful of the federal unions - called the president, their boss, a liar and voted to withdraw their support for his civil service shakeup plan. They censured Carter for proposing it and blasted him for "lying" to them about pay raises and other bread and butter issues.

White House officials were jolted when they got the word. They have been using this union's support to win over pro-labor members of Congress who would otherwise shy away from the reforms which include making it easier to fire people as well as setting up a new system to give political appointees more control over the government's 9.200 top career executives.

The AFGE turnaround on the reform proposal could cost the bill many votes in the Senate and House when it comes up for action later this month.

The overwhelming rejection of the reform bill, and the hot and often nasty debate about it, also spell trouble for the union's president, Kenneth T. Blaylock. He faces a tough reelection fight here Wednesday. Blaylock has campaigned heavily on the fact that he and Carter worked closely on Civil Service reform.The vote against reform is all the most significant because the motion to censure Carter for it, and for limiting federal pay raises, was pushed by Blaylock's chief rival. He is Royal Sims, a national vice president from the Philadelphia region.

Everyone expected Civil Service reform to be the major issue at this convention, but nobody figured on a total rejection, and an early one at that. Nor did anyone expect that AFGE - the only major union supporting it - would react to bitterly.

Some observers now rate Sims, who came to the convention as an underdog, as now having an even chance to unseat Blaylock. Blaylock is 49, white, and a former Defense Department worked from Alabama. Sims is 10 years older, black, and was a Veteran's Administration worker before winning national office with the AFLCIO union. The third candidate, Chicago vice president Alan H. Kaplan, also opposes the Civil Service reform plan.

The telegram censuring the president, from the union which represents 600,000 of government workers, said Carter has backed off campaign promises to support labor-management reform and to help repeal the Hatch "no politics" Act for federal workers. It also blasts him for putting a 5.5 percent lid on the October federal military-federal pay raise and for trying to cut the pay of blue collar workers.

"Whereas, his actions since his inauguration have clearly demonstrated that he has lied not only to federal workers but to the American people as a whole . . ." the motion reads, "therefore be it resolved that this body censures Jimmy Carter, president of the United States, for his blatant lies to federal workers and the American people . . ."

Earlier in the day, Rep. Morris K. Udall, (D-Ariz.), normally a crowdpleaser at federal union meetings, was heckled for allegedly interfering in union politics. Udall, the power broker between labor, business and federal bureaucrats in the Civil Service reform fight, ran into trouble when he praised Blaylock. He said the union leader was farsighted for backing reform instead of opposing it. Udall said the mood of the country and the Congress is antibureaucratic and that "tough" reforms are coming. He said Blaylock and the AFGE were wise to get in on the bill-writing to get the best deal possible for the government's nearly 3 million workers.

Rep. Gladys N. Spellman, (D-Md.), got a better reception from delegates but also drew grumbles when she spoke kind words about Blaylock - something that visiting Washington dignitaries normally do at all union conventions.

Today the union members plan a mass march to the federal center here. They will protest the 5.5 percent pay raise, lack of a federal labor-management law and Civil Service reform. Because of the heavy media coverage here - Washington and Chicago newspapers, the two wire services, and both NBC and ABC television - White House officials fear what could have been a relatively symbolic bit of blowing off steam may have a major impact on the way Congress handles the Civil Service reform which Carter says is the "centerpiece" of his entire government reorganization plan.