President Carter, hoping to get back into the good graces of organized labor, will back the AFLCIO's top-priority legislative proposal to make it easier for unions to organize and win contracts.

Administration and union sources disclosed yesterday that Carter approved the basic outlines of a labor law revision package after the AFLCIO agreed to a number of concessions, which included dropping its proposals for repeal of state right-to-work laws.

Some details reportedly remain to be worked out, but the White House confirmed that a message on the subject will be sent to Congress within 10 days. Another source said a message is expected by Friday.

Carter's agreement to support union-proposed labor law changes, reached after extensive top-level negotiations, represents a major victory for organized labor, which hasn't fared too well this year in either the White House or Congress.

"We're very much encouraged," said Albert J. Zack, the AFL-CIO's public relations director.

Administration backing has been considered crucial to the package's fate in Congress, which jarred the AFL-CIO earlier in the year by junking legislation to expand picketing rights on construction sites. Congress had earlier passed the provision, but it was vetoed by President Ford.

But passage of the package is by no means assured. The same management groups whose lobbying helped doom the picketing bill have already regrouped and started raising funds to defeat labor law revision.

According to administration and union sources, key elements of the labor law package include:

Strict deadlines, reportedly ranging from 15 to 75 days, for scheduling a union certification election after authorization cards have been signed, thus eliminating the long delays that unions charge can be used to pressure employees to vote against unions.

Payment of double back wages to employees illegally fired for engaging in union activities, an effort to make such firings too expensive to be worthwhile.

Denying federal contracts to employers who willfully violate labor laws, with flexible language to cover contracts deemed important to national security.

Increased compensation, based on industry averages, for workers whose employers refuse to negotiate with a legitimate bargaining unit, another effort to prod reluctant employers to the bargaining table.

Changes in the size and operations of the National Labor Relations Board which adjudicates labor-management disputes, including increasing its size from five to seven members and permitting two members, rather than the full board, to decide routine cases.

A requirement that the NLRB issue broad rules for definition of bargaining units, rather than making the definitions on a case-by-case basis.

Presidential support for the labor law package, which the AFL-CIO called its No. 1 legislative priority, followed a series of White House actions that chilled Carter's relations with organized labor since it helped elect him in November. Chief among these actions was his proposal for a minimum wage increase that unions described as stingy. However, negotiations with Congress on the minimum wage inc issue are continuing, and a congressional source said yesterday that chances for a compromise are "improving."

In addition to dropping its proposal for repeal of Section 14(b) of the Taft-Hartley Act, which permits state right-to-work laws banning union shops, the AFL-CIO abandoned a proposal for automatic certification of a union bargaining unit if more than half the affected workers sign authorization cards. The White House insisted on retention of secret-ballot elections, agreeing instead to a speed-up in the election process, which was labor's fall-back position anyway. Other union concessions included agreeing to double rather than triple back pay for illegal discharges and seven rather than nine. NLRB members, sources said.