Across East Texas yesterday, a professional radio announcer, a crisp urgency in his voice, repeated the same message every hour.

"To help President Reagan stop communist expansion in Central America, you need to make one important phone call," he said. "Call your congressman, Jim Chapman, today. . . . "

The 30-second commercial, paid for by the American Defense Lobby, began running on many of the about 30 small radio stations that blanket the First Congressional District of Texas yesterday morning and will continue today. The target is Chapman, a Democrat elected to fill a vacancy in a special election last August, one of about 75 key lawmakers who will cast the decisive swing votes in the House on Reagan's request for $100 million in military and humanitarian aid for the contras, or counterrevolutionaries, in Nicaragua.

The radio commercials are only a small part of the intensive lobbying campaign on the contra aid issue that is now building toward a climax with the House vote scheduled Thursday. The president's nationally televised speech Sunday night, in which he urged a flood of messages to Congress in support of the aid package, was the kickoff of the final drive by activists on both sides of the emotional dispute.

Despite Reagan's television appeal, House Democratic leaders said yesterday they continue to maintain a majority against the aid request. "We're ahead, and there is no perceptible erosion," said House Majority Whip Thomas S. Foley (D-Wash.). A Republican leadership aide did not dispute this but added that the Reagan speech was certain to help the administration's cause.

There is also a general perception that Reagan has made recent gains, especially among wavering House Republicans, tightening the vote count that Democratic leaders said last week showed a 12- to 15-vote margin against the aid request. But Foley said late yesterday that the Reagan speech "has not impacted in the House at all, and I don't expect it to. My very great confidence is that we will win this vote on Thursday."

The Reagan speech, however, did have at least one intended effect -- a flood of messages yesterday to Capitol Hill. Late last month, the president gave a similar nationally televised appeal for his defense spending proposals that fell flat, generating little pressure on lawmakers.

But on the contra aid issue, the administration and its allies were poised for an aggressive follow-up campaign, while the speech also appeared to energize opponents of the aid package.

The result was clear in the office of Sen. Arlen Specter (R-Pa.), which by late yesterday had received 773 calls in favor of aid to the contras and 673 opposed. After the defense spending speech, Specter received 41 calls, all opposed to higher defense spending.

Brian Detter, an aide to Chapman, said he could not recall any calls after the defense speech. But by early yesterday afternoon, with the radio commercials playing in the district, Chapman had received 69 phone messages urging support of contra aid and 34 against.

"Nothing has ever equaled this," Detter said.

Most of the pressure on Chapman is coming from supporters of contra aid, for whom the freshman lawmaker is an inviting target. He won a tough, special election that attracted national attention last summer in a district that historically is bedrock Democratic and deeply conservative, a district the GOP has realistic hopes of capturing some day.

He is also a southerner, and it is southerners of both parties who are giving Reagan his strongest support on the issue.

Finally, Chapman is undecided on the issue. "He's wide open," said Detter, a position that assures Chapman a maximum amount of attention over the next 72 hours.

The high-level attention to this obscure, freshman lawmaker began in earnest yesterday morning with a telephone call from Vice President Bush. It was one of about a half-dozen calls Bush made to key House members yesterday, according to Marlin Fitzwater, the vice president's press secretary. In addition, Bush is meeting with House and Senate members each day he is on Capitol Hill, Fitzwater said.

Today at 11 a.m., Chapman is to receive in his office a briefing on Nicaragua from two officials of the Central Intelligence Agency. Two hours later, he will be among 20 House members invited to meet with Reagan at the White House.

Meanwhile, other groups are working on Chapman and his swing-vote colleagues. One, Citizens for Reagan, yesterday had delivered to congressional offices a large poster showing Nicaraguan leader Daniel Ortega in the company of Libya's Col. Muammar Qaddafi.

The poster included a quotation from Qaddafi: "Libyan fighters, arms and backing to the Nicaraguan people have reached them because they fight with us. They fight America on its own ground."

According to Ken Boehm, chairman of the group, the posters were a small part of his organization's $250,000 campaign for the aid package that includes targeted mass mailings, newspaper advertisements, phone banks and other modern devices to influence public opinion.

Although they cannot match the resources of the administration and its allies, opponents of the aid package are also stepping up efforts. This week -- the week the House leadership scheduled for the key vote -- happens to be "Central America Week," when the Interreligious Task Force on Central America had long scheduled activities around the country in opposition to administration policy.

Members of Congress were sent letters yesterday signed by eight religious and private humanitarian organizations urging them to reject the humantarian aid portion of the package, $30 million of the total $100 million. The eight said the aid does not meet "the customary test" for humanitarian aid and "mislabeling [it] imperils the integrity of bona fide humanitarian aid."

The new presiding bishop of the Episcopal Church, Edmond Browning, also urged Congress to reject the aid. He cited resolutions adopted by the 3.1 million-member denomination's general convention in opposition.

Many of the lobbies are well organized and sophisticated. Chapman, for example, maintains four local offices in his sprawling district, but only one, in Sulphur Springs, is open every day. The radio commercials provided the phone number of the Sulphur Springs office to contact the local congressman.

Still, in even the most sophisticated lobbying campaign, mistakes will be made. Last week, John T. Dolan, chairman of the National Conservative Political Action Committee, wrote to Chapman urging support for aid to the contras.

"Your negative vote effectively drives a nail into the coffin of liberty and democracy for the Nicaraguan people," wrote Dolan, who enclosed a nail to dramatize the message.

Dolan had told Chapman "to reconsider your opposition" to contra aid. The Texan was not a member of Congress the last time the issue was voted on last spring. Chapman replied, "As so often happens at NCPAC, you have 'screwed up' with erroneous information."

He enclosed a screw.