Viewers of the 6 o'clock news on Los Angeles's KABC-TV last week met a public official they had probably never seen before, Leslie H. Gelb, director of the State Department's office of politico-military affairs. In an interview broadcast in segments on Wednesday and Thursday, Gelb was promoting the new strategic arms limitation those SALT pacts are still not signed.

It was a local news show, but KABC-TV had exclusive videotape of Gelb speaking from his Washington office. The State Department made the videotape and shipped it to Los Angeles for broadcast.

This is the new merchandising of SALT, the issue that is likely to dominate 1979 in the Senate, the issue that many in the White House regard as second only to inflation in building a record for President Carter's reelection campaign.

Although final agreement on a new SALT package is not now expected before January at the earliest, the administration has intensified its marketing campaign in recent weeks in preparation for next year's Senate debate.

A high-level SALT committee has begun regular meetings in the situation room in the White House basement to begin plotting the sales campaign for the new agreements.

At the State Department, a "SALT working group," led by an official who performed a similar function in promoting the Panama Canal treaties is organizing conferences for "opinion leaders" around the country to extol the benefits of SALT.

The department's bureau of public affairs has set up a speaker's bureau to make avaiable senior officials (like Gelb) for speeches or interviews.

"There's been so much misinformation put out about the SALT agreements" by their critics, one State Department official said last week, "that we have had to begin correcting the public record."

At the Pentagon, administration officials are engaged in another for mof alesmanship for the arms control agreements. They are drawing up plans for a long-range defense program invoving substantial new strategic weapons doubtful senators that the Carter administration is serious about matching Soviet strategic programs.

According to administration officials, President Carter is likely to announce a long-range defense plan at the time a new SALT pact is finally signed with the Soviets.

According to an authoritative source, the White House intends to respond to the charge - made by hard-line critics of the SALT agreements, and disputed by others - that the United States has not built any new "strategic systems" since the 1960s, while the Soviets have deployed several new systems. This will apparently mean intensified commitments to completion of the next MX missile, a land-based intercontinental ballistic missile, and a Trident II submarine-based missile.

The Pentagon has also undertakes a public relations campaign on behalf of the major new weapon system of the Carter era, the cruise missile. Last week, for example, the Pentagon's chief of research called a press conference to praise the cruise missile, an unmanned, subsonic jet drone that can read the earth's terrain to reach targets accurately. He described it as virtually invincible to Soviet defenses in the foreseeable future.

By January, adminstration officals plan to have an extensive SALT marketing campaign in full swing. By then it will involve Anne Wexler, the White House aide who has earlier mobilized numerous interest groups on behalf of President Carter's energy and public works programs, and Gerald Rafshoon, Carter's media adviser.

Though SALT remains a hostage to unforeseeable events, many of the perils any new agreements will face in the Senate are already becoming clear.

From the administration's point of view, the gravest apparent danger is that opposition to the SALT agreements could solidify at an early stage. This could happen if a consensus against the pacts emerges among the 41 Senate Republicans.

The administration hopes to prevent this by cloaking the new agreements in the drapery of peace and common sense, and depicting their critics as reckless proponents of a new arms race. A central administration argument for SALT, officials say, will be predictions of the dire consequences, particularly for the defense budget, if the agreements are not approved.

"We'll scare the hell out of them," an official said, describing the likely scenario. He predicted that the president will able to raise a plausible spectre of billions of dollars in new arms expenditures without SALT, with no guarantee that those expenditures would produce enhanced national security.

But opponents of the agreements said in interviews last week that they doubted Carter's ability to seize the high ground in the SALT debate, since so many respectable critics, including senior officials of the last four administrations, can be expected to speak out against the new pacts.

Moreover, the opponents say, they do not have to come out against SALT altogether, or to sabotage the pacts now being negotiated.Instead, as one Senate aide noted, the opponents can look for apparently modest modifications to the agreements that in fact the Soviets would find unacceptable, effectively killing them.

The Senate minority leader, Howard H. Baker Jr. (R-Tenn), is again likely to be a crucial figure, as he was in the debate over the Panama Canal. Then he supported the president, making possible the two-vote margin by which the canal treaties won Senate approval.

In his recent reelection campaign Baker took a hard line on SALT, though without declaring outright opposition. Baker is an admirer of Lt. Gen. Edward L. Rowny, the Joint Chiefs of Staff's representative on the SALT negotiating team, who is known to be deeply skeptical of the agreements that are now nearly complete. Rowny's role is a source of great concern inside the administration, where it is feared he may - overtly or covertly - work against the SALT agreements.

A presidential hopeful like Baker, armed with argumentation from Rowny and the enthusiastic support of conservatives whom he alienated in the Panama debate, could come out for crippling changes in the SALT pacts, and he could take other moderate Republicans are counted as certain votes for SALT next year, 31/2 "no" votes can block the agreements.

The Baker problem is an example of the difficulties that may lie beyond the reach of the administration's marketing campaign. Officials acknowledge that the outcome of the SALT debate could hinge on Carter's reputation and political standing at the time of a final vote, rather than on any specific aspect of the agreements.

Meanwhile, administration lobbyists - who feel that their big foreign policy victories in Congress's last session prepared them well for the SALT showdown - are plotting their most ambitious effort since Carter became president.

Next month the administration hopes to brief all of the 20 newly elected senators on SALT. The heavy turnover and the loss of five known SALT supporters in the balloting this month created new uncertainties for the administration. (Official votecounters think the net effect of the election may have been a loss of two SALT votes in the senate, but this is a guess)

By the time the Senate debate begins, the administration hopes that a group of pro-SALT senators led by Alan Cranston (D-Calif.), who have been working for months on SALT issues, will be up to a confrontation with Henry M. Jackson (D-Wash.), knowledgable leader of the hard-liners.

Adminstration officials are pleased with their apparently ability to improve the "management" of SALT matters in the last few months. Last spring, officials now acknowledge more freely than many did then, decisions were taken to "toughen" the U.S. bargaining position and slow down the SALT process because the Senate climate then seemed so unpropitious.

Now, thanks to a change in the mood of Soviet-American relations, the Camp David summit and other factors, officials perceive a more friendly climate for SALT, though most acknowledge it could disappear quickly.