After months of hesitation, the White House has finally decided to engage President Carter directly and personally in lobbying senators for the new strategic arms limitation treaty (SALT).

As a result, Carter has held a series of meetings with key senators, individually and in groups, over the last few days. Yesterday, for example, the president had breakfast with the newly elected senatirs, met for an extensive discussion with 12 senators who visited the Soviet Unon late last year, and then talked alone for 75 minutes with Sen. Henry M. (Scoop) Jackson (D-Wash.), a leading member of the Armed Services Committee and often a critic of the SALT process.

Earlier this week or last Carter met with Sens. John C. Stennis (D-Miss.), chairman of Armed Services, and Sam Nunn (D-Ga.), a member of the committee who is expected to play an intluential role in the SALT debate.

Carter has apparently reiterated in all of these sessions that he regards the new SALT agreement now being negotiated as a matter of crucial importance to this country and the cause of world peace. But in reply he has often heard what must have seemed like bad news.

For example, Sen. Jacob Javits (RN.Y.), one of the 12 senators who traveled to the Soviet Union in a delegation led by Sen. Abraham Ribicoff (D-Conn.), advised Carter not to initial the new SALT pact with the Soviets until he was certain that at least a majority of the Senate was prepared to support it.

"That was a real shocker to Carter," another senator present at the meeting said last night.

Javits' implication that Carter could not be sure of majority support meant the president was far indeed from the two-thirds Senate approval that will be necessary approve a SALT pact with the Soviets.

Ribicoff, an avowed supporter of SALT II, told Carter that if the Senate voted today, there would not be a two-thirds majerity for the agreement. But he added that the approval of 67 senators could still be obtained.

Senators sy-npathetic to SALT and to Carter have complained for more than a year that the administration has been insufficiently active in making a case for its arms limitation policies, leaving the field open to its opponents.

According to several administration sources, the president's senior advisers decided earller this month that the prospects for SALT were sufficiently uncertain that they had to accelerate their lobbying efforts immediately.

Previously, the White House had taken the position that it should not lobby too intensively until a final agreement with the Soviets was in hand. The process of negotiating that agreement has now dragged on for more than six years, thropugh three administrations.

There are wide differences of opinion within the highest levels of the Catrer administration as to the SALT pact's chances in the Senate. Some officials argue that when the ageement is finally before the Senate, the benefits it offers and the gravity of rejecting it will combine to win the necessary two-thirds.

Others involved in the SALT-selling process are much more gloomy, reportedly arguing that an all-out political campaign will be required to win Senate approval for SALT II.

The latter view seemed to be reinforced yesterday when Crater met the Ribicoff delegation. According to many of the senators present, they all expressed reservations about SALT, many told Carter they were concerned about "linkage" between the arms agreements and other aspects of Soviet-American relations.

Ribicoff told Carter he should use the period of months between the initialing of SALT and final Senate action to try to work out understandings with the Soviets on issues that trouble the Senate, such as the Soviet-Cuban presence in Africa. Other senators said the full range of Soviet behavior would inevitably affect many senators' votes on SALT.

"It would have been better if this message [on linkage] had been absorbed months ago," one senator who has seen the president recently said last night. He noted that now the Soviet and U.S. governments agree that SALT should be judged independently of other issuses, while the U.S. Senate seems to feel that this is politically impossible.

Sen. Jackson declined to discuss his private conversation with the president, except to say that it covered many issues besides SALT and was "very frank."