More congressmen back Reagan on defense in wake of jetliner downing

President Reagan, who has been preaching to a reluctant Congress for months in favor of a massive defense buildup, will find more believers next week when the lawmakers return from their August break.

That is the assessment of members of both parties in the wake of the shooting down by the Soviet Union of a South Korean passenger plane. Not only did the President's speech Monday night win high marks for moderation on Capitol Hill, but he seems finally to have found a persuasive argument for his defense program.

After bickering with Congress over defense growth and whether to build the MX missile, Mr. Reagan now appears to have ensured short-term victories.

Congress is scheduled to act next week on a House-Senate conference report authorizing the construction of the MX, as well as the B1 bomber and new nerve gas weapons, all of which have been controversial in the past.

''Now we'll get more votes for the MX,'' says Rep. Les Aspin (D) of Wisconsin , a member of the Armed Services Committee who backs the missile. ''My guess is the authorization bill will be easy'' to pass, he says.

The Korean airline incident ''puts a new emotional element into the equation, '' concedes Rep. Jim Leach, an Iowa Republican who opposes the MX and who has helped lead the nuclear freeze movement.

Representative Leach predicts a victory for the MX, which only narrowly survived its last vote in the House. ''Congress will have a very difficult time turning it back,'' he says.

Rep. Newt Gingrich of Georgia, a conservative Republican who criticizes the President for not imposing more stringent sanctions against the Russians, says that ''liberals and radicals'' will find it harder to blame the US arms buildup for threatening world peace.

He predicts that Congress will pass a ''somewhat higher appropriations'' bill for defense than it would have.

However, defense spending for the year beginning Oct. 1 has already been set at 5 percent real growth, compared with 10 percent originally asked by the White House. An aide to Senate Budget chairman Pete V. Domenici (R) of New Mexico says the spending bill might go up slightly, to about 5.5 percent, when it is enacted this fall.

The next real opportunity to boost defense spending markedly comes next year, by which time the current outrage would have faded. ''I think we're going to be in an anticommunist mode for the next couple of months,'' says the Domenici aide.

''It's hard to imagine that this is going to have a lasting effect,'' says Representative Aspin, who sees ''for the next couple of weeks a more hawkish mood.''

Sen. Carl M. Levin (D) of Michigan, a member of the Senate Armed Forces Committee and a staunch MX opponent and nuclear freeze advocate, does not concede defeat yet. ''This incident proves the nature of the Soviet bear, but also that you don't want to take any risks with the Soviet Union,'' says Senator Levin, arguing that the big, 10-warhead MX missile is too tempting a target for the Russians.

Other supporters of the nuclear freeze say they are still taking stock of events. The freeze resolution has passed the House and is scheduled to go before the Senate Foreign Relations Committee later this month.