In a series of policy shifts in recent months, President Carter has strengthened his position on what promises to be one of the most sensitive issues in the fall campaign: the Republican charge that he is "soft" on defense.

Though aides say the reason for the shifts were substantive, not political, the new policies nevertheless leave Repbulican nominee Ronald Reagan confronting a much-changed Jimmy Carter in the defense field.

For example:

After braggin early in the administration that they had cut Republican defense budgets, Carter and his teammates are now taking credit for raising them.

After scolding the Joint Chiefs of Staff for lobbying so hard and so publicly for higher military pay, Carter is calling for higher benefits and showing every sign of going along with congressionally approved pay raises for Army, Navy and Air Force personnel.

After vetoing one military weapons bill because it authorized a nuclear aircraft carrier, he signed the next one -- carrier and all.

After delaying the pace set by President Ford for the MX missile, Carter is now calling for full speed ahead.

Advances in weapons, together with a maturing policy for employing them, also have put Carter in a strong position to defend his controversial cancellation of the B1 bomber earlier in his administration, and his strategic nuclear policy.

Vice President Mondale, in his acceptance speech at the Democratic National Convention last week, gave a glimpse of how he and Carter intend to wage the battle of the B1. Mondale said the B1 would not have stood up against Soviet defenses if the President had built it.

"Mr. Reagan," Mondale said, "scolds us for not having an outmoded bomber that would be obsolete and vulnerable the day that it was launched."

The Pentagon has several bombers on the drawing board that advocates consider better than the B1, including a Ghost plane that enemy radar could not find. Carter is in position ot embrace an advanced bomber any time he feels defensive about his B1 decision, already assailed by the Republicans. Congres is insisting he choose one no later than March.

Carter recently signed off on a new nuclear policy, Presidential Directive 59, which embraces what many conservatives, including some Reagan advisers, have long been advocating: more emphasis on trageting Soviet command and control centers and military facilities.

Carter also can point to the Rapid Deployment Force being organized to respond to crises in the Persian Gulf and other distant spots.

To help him explain and defend those and other defense policies during the election campaign, Carter has enlisted a seemingly willing Harold Brown, his secretary of defense. Brown, a scientist who helped develop the H-bomb, is unrivaled in the Reagan camp for expertise in modern nuclear weapons.

Brown departed from the dispassionate tone of the scientist last month in assailing the Republicans' call for military superiority over the Soviet Union, calling such a pursuit "unrealistic, simplistic, dangerous."

Also, Brown flew to the Democratic National Convention last week to press for the fast-paced MX missile development Carter advocates. Air Force Undersecretary Antonia Chayes was also in New York fighting for Carter's MX plank.

Former defense secretary Melvin R. Laird, former Republican Party leader, in a Washington Post column on Sunday said Brown's attack on GOP defense budgets were "distorted" and warned against "politicizing the office" of secretary of defense.

The public record documents a reversal by Carter and his administration on how much is enough for the national defense.

"Without endangering the defense of our nation or our commitments to our allies, Carter wrote the Democratic Platform Committee on June 10, 1976, "we can reduce present defense expenditures by $5-to $7 billion annually."

Defense Secretary Brown, on Feb. 21, 1977, said the administration had started to make good on that pledge by cutting the fiscal year 1978 budget inherited from former president Ford by $2.8 billion. In briefing reporters on Jan. 23, 1978 on Carter's first defense budget -- the one for fiscal 1979 -- Brown said the administration had cut the budgets planned by Ford.

"The budget that President Ford submitted for fiscal year 1978 was $4 billion more than the budget that this administration submitted to the Congress for fiscal 1978," said Brown back then. "The budget in these very documents of last year," the Ford projection of future defense budgets, "that the Ford administration proposed for fiscal 1979, was $8.4 billion more than we are now proposing, in obligational authority and $5 1/2 billion more in outlays than we are proposing. Those are the reductions that this administration has accomplished."

After fighting congressional demands last year for increasing the defense budget more than 3 percent, the Carter administration has gone along this year with a real growth of about 5 percent. And its tone about inherited Republican defense budgets is decidedly different, with Brown asserting before the Commonwealth Club of California on July 28:

"When this administration took office, we inherited a military posture and a defense budget that simply had not kept pace."

In reviewing Ford's last defense budget, in 1977, Carter decided to slow down the pace of the MX missile to allow more time to study how it should be deployed. He has advocated a faster pace in subsequent years, capped by his handwritten plea to the Democratic National Convention delagates last week to back the MX to demonstrate "we are committed to defending our country."

Carter earlier this year sent Brown a memo rejecting pleas by the Joint Chiefs of Staff to approve big pay raises for military people, directing them to "assess other factors . . . When I was in the Navy, pay was not the major factor." The White House, when queried, did not deny Carter had sent such a memo.

On board the aircraft carrier Nimitz this past July 4 weekend, Carter said he agreed military benefits should be improved. Congress is putting together a benefits-pay package for military people that the president seems certain to approve -- a step that would plug one more chink in his political armor.