Gen. Alexander M. Haig Jr. is once again a leading candidate for the post of secretary of state in the Reagan administration, well-placed sources said yesterday, suggesting anew that the search for a Reagan Cabinet is something of a political roller-coaster ride.

At the end of last week the same knowledgeable sources were talking as though Haig's fortunes were fading fast after Republican leaders in the Senate warned that a messy confirmation fight was possible if Ronald Reagan did nominate the former chief of staff in Richard Nixon's White House.

Haig's resuscitation since the weekend typifies the ups and downs of the Cabinet selection process, a process that has upset some Republicans because of its apparent unseemliness.

A final, formal decision on Haig may be made in Washington today when the president-elect finally sits down at the same table with his principal associates to consider the remaining unfilled Cabinet positions. Haig has many supporters on the Reagan transition team who are urging that Reagan defy Senate Democrats who have threatened to pull Haig through Watergate and Vietnam mud if he is named secretary of state.

Reagan himself was apparently consulting by telephone on the Haig matter. On Sunday, for example, he had a long telephone conversation with former president Gerald Ford, according to an aide to Ford. Haig's role in the pardoning of Richard Nixon by Ford is a particularly sensitive issue in the nomination question.

Sen. Jesse Helms (R-N.C.), a leading Haig supporter, said in an interview yesterday that "if President-elect Reagan werre to flake on this first nomination, then he would send a message to the Democrats that they could shake him on every other nomination" later on. Helms predicted that a Republican-controlled Senate would have no trouble confirming Haig's nomination.

Sen. Claiborne Pell (D-R.I.), who will be the ranking Democrat on the Senate Foreign Relations Committee in the new Congress, said yesterday he agreed that Haig would be confirmed, but said his effectiveness as secretary of state could be damaged by a messy confirmation fight. Though "a singularly able, competent man," Pell said, Haig "carries with him the burden of being a military man . . . and Watergate.

"I don't see why the administration would want to start out with any excess baggage of that kind."

Other Democrats, including Robert C. Byrd (D-W.Va.), who will be the minority leader in January, have promised to give Haig close scrutiny if he is nominated. "I think the questions raised [by Haig's involvement in Watergate-related matters] are serious enough that it is possible he would not be confirmed," Byrd said Saturday.

Sen. John Glenn (D-Ohio), a member of the Foreign Relations Committee, which would consider Haig's nomination, said yesterday he is "not one who's out to nail Haig," but promised a thorough inquiry into his past. Glenn said there were "two schools of thought on Haig," one that he escaped implication in the bad deeds of Watergate "by the skin of his teeth," the other that "he deserves a medal" for easing Nixon out of the presidency in August 1974.

Sen. Howard H. Baker Jr. (R-Tenn.), who will be the Senate majority leader in January, said yesterday he would actively support Haig for secretary of state if Reagan nominates him for the job. Baker predicted Haig would be confirmed. This was apparently a significant signal, since it had been Baker who cautioned the Reagan camp last week that nominating Haig might cause embarrassing trouble in the Senate.

One factor that helped revive Haig's prospects, sources in the transition team said yesterday, was the paucity of alternatives to him. "Look at the choices," one transition official said after predicting that "Haig is going to get it."

Alternatives mentioned by sources close to Reagan last Friday were George P. Shultz, Caspar W. Weinberger and William J. Casey. Shultz is hotly opposed by many conservatives, Weinberger is in line to be secretary of defense and Casey is set to become director of the Central Intelligence Agency. None of the three has extensive experience in foreign affairs.

Several sources in the Reagan operation predicted that Senate opposition to Haig would evaporate as soon as he was nominated. "Once he's nominated everyone is going to be for him. That's just the way the Senate works," one said. Said another transition official of Democrats in the Senate: "Who wants to be the guy who went after one of the major powers in this town and failed to get him? Especially if you're on Foreign Relations and this is the secretary of state?"

"It's all smoke and no fire," one of Reagan's senior advisers said yesterday of hints that Haig was somehow guilty of malfeasance during his earlier government service.

Well-informed sources revealed yesterday that Haig has remained in daily touch with senior Reagan aides. A spokesman for the retired general, now president of United Technologies Inc., in Hartford, Conn., has said repeatedly that Haig had heard nothing from Reagan about the State Department job. The new information about his daily calls to Reagan aides indicates that the full story was more complicated that that, however.

Some transition aides privately expressed criticism that the way the Cabinet selection process has gone on has allowed prominent Republicans' names to be dangled embarrassingly in public and sometimes subjected to harsh public criticism. "Reagan . . . is inviting guerrilla warfare" over his choices, one aide said.