The House last night approved a $157.5 billion appropriation for the Pentagon next year that rises $20 billion over current spending, reflecting both inflation and a post-Afghanistan, pre-election push to strengthen the national defense.

In another indication of the pro-defense mood of Congress, the Senate joined the House in sanctioning initial steps toward resumption of nerve gas production for the first time in a decade.

At the same time, routine preparations by the House Appropriations Committee for a stopgap measure to keep the government running after the fiscal year ends Sept. 30 were stalled because of apparently strong pressure to increase defense spending to 1981 levels.

Normally such a continuing resolution, required when Congress is laggard in passing its money bills as it is this year, keeps spending at the previous year's levels. The Appropriations Committee decided to put off action and regroup because of concern that the defense spending increase, advocated by Rep. Norman Dicks (D-Wash.), would prompt demands for increases in other areas as well, according to a committee source.

With only about 10 working days until the fiscal years ends, "it could get awfully messy," the source said.

Before approving the huge defense money bill, which contemplates a 50 percent increase in Pentagon spending since 1977, the House rejected a proposal to delay the controversial MX mobile missile-basing system for further study. But it agreed to lift a longstanding ban on targeting defense money into economically depressed areas.

It also added $100 million to the $1 billion that the Appropriations Committe approved for armed forces recruiting.

The targeting proposal, advanced by defense appropriations subcommittee chairman Joseph P. Addabbo (D-N.Y.) and approval 220 to 179, would permit the Pentagon to pay higher prices for up to $15 billion in procurement contracts in order to channel work into areas of high unemployment.

On chemical warfare, the House handily rejected, 276 to 125, a move to scrub $19 million from the defense money bill for equipment for nerve gas production. This occurred just as the Senate was following the House's lead in authorizing $3.2 million for a nerve gas plant in Pine Bluff, Ark.

But the Senate vote indicated some misgivings about this new breed of nerve-gas warfare involving so-called binary chemicals, which are theoretically harmless until mixed in firing to become lethal agents.

But only 47 to 46, it rejected a compromise proposed by Sen. Gary Hart (D-Colo.) for a full scale executive branch study of chemical warfare and for Congress to take up the issuue again after March 1. Then it approved the authorization, 52 to 36.

In debate in both houses, it was argued that the initial funding would not commit the country to producing nerve gas but would be valuable both as a battlefield deterrent and a bargaining chip for arms control negotiations. a

"In fact inferior American chemical warfare capabilities and American unilateral constraint . . . have failed utterly to move the Soviets closer to a genuine, reciprocal chemical weapons agreement," argued Sen. Henry M. Jackson (D-Wash.) on behalf of the Armed Services Committee.

The Carter administration had proposed no funds for chemical warfare preparations and opposed the authorization.Among congressional opponents, Rep. Toby MOFFETT (D-Conn.) argued that spending for chemical warfare was a 'provocative move" that would "send a horrible signal" to the rest of the world. Rep. Margaret Heckler (R-Mass.) said it was "unworthy of this nation."

The defense appropriations bill, which was approved on a final House vote of 351 to 42 and now goes to the Senate, is $2.5 billion more than the adminstration sought and contemplates passage of an 11.7 percent military pay raise in a supplemental bill that will cost $3.7 billion or more. This is in addition to $713 million in the approved bill for military personnel benefit increases.