Money, workers and expertise are pouring into Alabama from the national liberal political apparatus to win a Senate race because of an issue scarcely discussed there: ratification of the prospective SALT II treaty.

Well-placed liberal leaders claim an iron pledge from State Sen. Donald Stewart to vote for the new arms control pact if elected to the U.S. Senate. Stewart denies any commitment on SALT and is publicly critical of what Soviet-American negotiations have produced. But that bothers his New York-based liberal backers not at all. "That's what you have to say to get elected in Alabama," one told us.

The question then arises of just who is fooling whom. Is the good old country boy fleecing the city slickers from Manhattan? Or is it Alabama's hawkish, anti-communist electorate that is being deceived? Nobody can be sure until such time as Stewart votes on SALT in the Senate.

What is happening in Alabama is duplicated in other Senate races. Whether President Carter's SALT II wins the necessary two-thirds Senate vote next year may well depend on a dozen Senate races Nov. 7. But the hideously complicated questions of arms control, vital to the nation's security, are not discussed at all by Senate candidates.

Even when asked, few candidates give straight SALT answers. The result, therefore, is a lottery. While the real vote on SALT II may be by the people Nov. 7, in many states they have no way of knowing which side they are picking. That is particularly true of Alabama's election to fill the two years remaining in the term of the late senator James Allen, who was a sure vote against SALT II.

SALT was no issue when Stewart, 38, emerged from obscurity to upset Maryon Allen, the senator's widow, in the Sept. 26 Democratic runoff. But the National Committee for an Effective Congress (NCEC), a long-established liberal political-action organization, quietly put nearly $5,000 into Stewart's campaign mainly because of that issue.

Responsible liberal sources report that Russell Hemenway, the astute Manhattan liberal politician who runs NCEC, had flat assurances from Stewart of voting yes on SALT. Gaining an arms-control vote from the Deep South generated celebrations in New York and Washington the day after the Alabama runoff.

But Alabama Republicans, sensing that a previously unattainable seat had become possible, shuffled their slim deck and pulled out a new opponent for Stewart: former representative James Martin, a little shopworn since nearly upsetting then-Sen. Lister Hill in 1962. Martin is still Alabama's most potent Republican - potent enough to interrupt liberal celebrations up North.

The hard-charging Hemenway dispatched political operatives from elsewhere in the South to help Stewart and mustered another $5,000 for him. Though Stewart is cool to labor legislation, liberal unions are helping in no small part because of SALT.

Stewart's campaign literature reads as anti-SALT as Scoop Jackson, contending that the new agreement "leaves too much American defense on the [bargaining] table." Stewart reiterated that position to us, contending it is "not at all true" that he made any commitment and that "I have real concerns" about SALT II. Nevertheless, his liberal supporters claim to be not at all worried about such statements for home consumption.

Moreover, liberal leaders are listing as sure SALT senators several other Southern Democratic candidates - including Andrew Miller in Virginia and John Ingram in North Carolina - on the basis of private commitments. The candidates are publicly opaque. When we asked Ingram, he said he did not know enough about SALT to discuss it.

Then there is the mysterious case of Sen. Edward Brooke, the liberal Republican from Massachusetts threatened to equally liberal Democratic Rep. Paul Brooke against a right-wing Republican primary challenge because of tacit assurance that he opposes SALT II. But that word is laughed off by Democratic State Rep. Barney Frank, a Brooke supporter who persuaded the liberal Americans for Democratic Action to endorse both Brooke and Tsongas, not just Tsongas. "I am sure Brooke will be on our side [on SALT]," Frank told us.

Such confusion and obfuscation is possible because the treaty will be initialed after, not before, the midterm campaign. With no Soviet-American agreement in open view, the most important issue to be decided by Nov. 7 ballots is hidden. What results is Alabama's contrast between public statement and private expectation.