Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) predicted yesterday that Congress will cut more than $15 billion from the $247 billion military spending budget President Reagan had originally projected for fiscal 1984.

The $8 billion in budget outlay cuts announced by the president on Jan. 11 is not enough, Baker said on "Meet the Press" (NBC, WRC). "If we're ever going to get the budget under control, we're going to have to show a willingness to sacrifice in almost every sector of the federal budget, and military simply can't be immune from it," he added.

Republican leaders for weeks have been complaining that the White House has not seriously trimmed the Pentagon's budget, but Baker's assessment that the GOP-controlled Senate would almost double the president's cuts was the strongest indication to date that Reagan is unlikely to get his way on military spending this year.

The $8 billion in cuts that the administration had announced would come mostly from lower fuel prices, due to the decline in inflation, and from forgoing the 7.6 percent military pay raise scheduled for October. No major weapons systems would be delayed under Reagan's budget, despite opposition in Congress to the MX missile.

Congressional leaders point out that, even with Reagan's $8 billion in outlay cuts, military spending under his 1984 budget would increase by 14 percent, while domestic programs would continue to be cut. In calculating cuts, the administration started with Reagan's original five-year military buildup figure, which projected $247 billion in military spending for 1984.

Congress, however, is using the fiscal 1983 military budget it approved last year as a basis for determining cuts. Under that budget, the Defense Department will spend $208.8 billion in fiscal 1983. Budget authority for 1983, which includes spending over the life of a weapon's program, is at $238.5 billion.

On the domestic side of the budget, Baker said he opposed the administration's idea for a tax increase triggered to take effect two or three years from now, depending on economic conditions. "I've given a lot of thought to that," he said. "It is my personal view that that's not a very good idea."

Baker also reiterated his opposition to the indexing of tax rates to the rate of inflation. Congress should adjust tax brackets from time to time to account for inflation, he said, "but to put it on automatic pilot . . . builds in economic and political distress that we're not prepared to cope with."

Despite his criticism of the Reagan budget, the Tennessee senator, who announced last week that he would not run for a fourth term in 1984 but would prepare for a possible presidential bid in 1988, went out of his way to praise the president.

"I got to confess to you I was never a Reagan admirer until he became president," he said. "I ran against him and said some pretty harsh things against him. But I think I was wrong. He's capable. He's alert. He is in control."

In an effort to deflect speculation that he might be positioning himself to run for president in 1984, Baker said, "As far as I'm concerned, 1984 is Ronald Reagan's year. He ought to run for reelection . . . . But if he were not to run in 1984, of course, I'd take another look at it." Picture, Sen. Howard H. Baker Jr . . . "We're going to have to show a willingness to sacrifice . . . " UPI