WASHINGTON (AP) _ In Congress' first legislative response to President Bush's anti-drug campaign, a Senate effort to bolster treatment programs by cutting funds for congressional mailings was halted Wednesday by a single senator.

The measure, proposed by Republicans and accepted by Democrats, would slash the $80 million pot proposed for congressional mail for next year to $35 million. The other $45 million would be transferred to a treatment program for pregnant women addicted to crack cocaine and other drugs.

But a vote was blocked by Sen. Ted Stevens, R-Alaska, who argued that such a sharp reduction in the mail lawmakers send their constituents would deprive Alaskans of an important source of news. Stevens said many Alaskans get no daily newspapers and learn of news from Washington only from radio and brief television broadcasts.

''This is wrong,'' Stevens said. ''This is a nation that has been kept together by communications.''

Stevens' objections forced Senate leaders to postpone a vote until at least Thursday.

The measure was proposed by Sen. Pete Wilson, R-Calif., a supporter of tougher efforts against drug abuse, a long-time critic of the burgeoning costs of legislators' mailings and a candidate for governor of California.

''My purpose is not to denigrate the newsletter,'' Wilson told his colleagues. ''It is instead to point out that there is a far more urgent, a far more compelling priority making a claim upon those same funds.''

The proposal put Democrats in a difficult position by forcing them to choose between assisting an anti-drug program serving a vulnerable segment of society, or maintaining their ability to pepper their constituents with newsletters.

Sen. Harry Reid, D-Nev., chairman of the Appropriations legislative subcommittee, said Democrats would accept Wilson's proposal.

The proposal was offered as an amendment to legislation providing $2 billion for Congress' own expenses for the 1990 fiscal year, which begins Oct. 1. The House version of the bill contains no such reduction in funds for mailing, and negotiators from the two chambers will eventually have to decide its fate.

In his Tuesday evening speech presenting his $7.9 billion drug campaign, Bush proposed spending $300 million for treatment programs. He listed initiatives to help pregnant drug users as part of the effort, but did not specifically say how much should be spent for them.

''As important as newletters may be and notices for town meetings may be, are they as important as what the president outlined last night on the war on drugs?'' asked Senate Minority Leader Bob Dole, R-Kan.

Wilson said that 400,000 babies are born annually in the United States to drug-addicted mothers.

The program the money would go to, created in 1988, is supposed to spend up to $10 million annually for addicted pregnant women, especially for those who use crack. This year, only $4.5 million has actually been spent, Wilson said.

The legislative spending bill is perenially a battleground for fights between the House and Senate.

One of those disputes this year is over congressional mailing costs. The $1.6 billion House version of the bill, approved July 31, contained $124.7 million for mailing.

As approved by the Senate Appropriations Committee, the bill would provide $80 million for congressional mailings: $48 million for the House and $32 million for the Senate. The House has 435 members, while the Senate has 100.

The Senate bill would also reduce from six to two the number of district- wide mailings permitted each year, except for notices of town meetings. Legislators could send more, but would have to pay for them out of the funds each receives for general office expenses.

This year's congressional mail expenses are expected to total $53.9 million. But traditionally, lawmakers' mailings increase dramatically in election years, and 1990 is one of those.

Critics of congressional mailings have argued that they are often little more than taxpayer-assisted publicity for incumbents.

The legislation also establishes a health club, which would include exercise classes, for Senate members and employees.