Pentagon analyses and Soviet actions have provided President Carter with fresh arguments for going back off his campaign promise by putting the B-1 bomber into production.

Last year, he said taking that step would be wasteful of taxpayers' money."

Although he will still draw protests if he does the expected and orders a limited number of the bombers into production, Pentagon studies portray the B-1 as the most attractive option for holding up the third leg ot the "triad."

The triad is the Pentagon term for the three types of strategic forces for shooting nuclear weapons at the Soviet Union: bombers, land-based missiles and submarine based missiles.

Currently, the bomber part of the triad is the aging fleet of B-52s. These planes were built originally to bomb from high up, but since have had their wings strenghtened to enable them to fly into Russia at treetop level to elude anti-aircraft defenses.

The B-70, which was supposed to be the update of the B-52, was canceled by President Kennedy in 1961 because high-flying bombers had become too vulnerable to Soviet rockets, like the one that downed Gary Powers' U-2 spy plane over Russia in 1960.

In caneling the B-70, a young "Whiz Kid" heading Pentagon research, Harold Brown, promised Congress that his office would study ways to build a bomber that could survive modern defenses. The B-1, after several mutations, is the result. And Brown is now Secretary of Defense.

In putting together his recommendation to Carter on whether to order the B-1 into production, Brown commissioned a number of studies and announced publicly that his decision would hinge largely on the kind or air defenses the Soviets were expected to field in the 1980s and 1990s.

One Pentagon study concluded it would cost almost as much to renovate the current fleet of 350 B-52s as to buy 244 brand new B-1s, $18 billion compared to $24.8 billion. Adding the $3 billion that already has been invested in the B-1, the difference is even smaller.

Another suggested alternative to the B-1 is stuffing civilian transport planes, like the Boeing 747, with air-to-ground missiles that could be fired while the plane remained a sale distance from enemy defenses.

But here again Carter can point to Pentagon studies showing that Soviet fighters could shoot down comparatively slow civilian transports before they could launch their missiles.

As for the future Soviet air defenses that Brown said were key to his recommendation, Air Force intelligence officers see nothing that threatens to stop an unacceptably high number of B-1s.

The Soviets, they said, are a long way from perfecting the kind of lookdown radar their fighters would need to spot the low-flying B-1. And Soviet ground defenses, partly because of a lag in computerizing the system, do not look particularly woorisome to the U.S. Air Force.

The offense, in the latest test of American bombers versus Soviet anti-aircraft measures, proved way ahead of the defense. The Air Force said in its Christmas bombing of Hanoi in 1972, 15 B-52s were lost in 700 flights to the target and back. The Air Force puts its loss rate at 2 per cent.

However, the loss rate is 12 per cent if figured on the basis that 15 out of about 125 B-52s were lost in more Christmas raids of 1972.

The Soviets, by refusing to consider seriously the comprehensive arms control proposal Carter sent to Moscow, have provided the President with the argument that he needs to keep building the B-1 for bargaining power, even though all arms building is "wastefull" - as the said during the campaign.

Also, the Soviets have a fairly modern bomber in production, the Backfire, while the B-52 has been out of production for years.

It will be tempting for Carter to state that he will build up toward a force of about 150 B-1 bombers but will call off the program if the Soviets agree to pull back on modernization of their strategic arsenal as well.

Although a B-1 go-ahead by Carter would infuriate citizen groups allied in a "Stop the B-1" campaign, few leaders in the arms control community would join their protest. Arms controllers are focusing instead on stopping weapons like the highly accurate Mark 12 warhead and MX mobile missle, which they fear would upset the balance of terror by threatening the Soviets with a surprise first strike. Arms controllers do not put the B-1 in the "first strike" category.