A weary Jim Chapman, Democratic winner of a special runoff congressional election Saturday in northeast Texas, said early this morning that he had picked up the scent of victory at a rodeo parade just before the polls closed.

"Must have had a dozen people come up and say 'What's President Reagan doing telling us how to vote?" said Chapman, second-guessing his Republican opponent's saturation use of Reagan in radio and television ads in the final 48 hours of the hard-fought campaign.

"They all like the president, but their attitude seemed to be that he ought to be above politics," Chapman added.

The ads probably did not cut much either way in Chapman's 51-to-49 percent squeaker over Republican Edd Hargett in the 1st Congressional District election, a race that national GOP strategists had thought they would win.

But the reaction they evoked goes part way toward explaining why the $1 million GOP effort came up short and why party realignment in the rural South is likely to keep unfolding in paces, not leaps.

Reagan is far and away the GOP's most bankable asset in populist rural areas such as Texas' 1st District, but he can stretch only so far, at the risk of reducing himself to a mere partisan. Hargett's strategists sensed that, and until their final get-out-the-vote push, they used Reagan sparingly. He did not travel into the district.

But take Reagan out of the argument, and Republicans are forced to go mum on the subject of party labels. Their 1980 appeal -- "Vote Republican. For a Change" -- was supplanted in this campaign by the tag line, "This election isn't about politics and party, it's about honesty and integrity."

"They never said the word 'Republican' above a whisper," said Rep. Tony Coelho (Calif.), chairman of the Democratic Congressional Campaign Committee. "That's realignment?"

Democratic National Chairman Paul G. Kirk Jr. echoed that thought today, saying the election results demonstrated that "the talk about realignment is still only a Republican dream."

In a sense, the Republicans' strategy for winning was as telling to their claims of realignment as was the defeat.

Armed with polling data showing that Chapman, a former district attorney, had lots of baggage from an unsuccessful campaign for state senator last year, Republican operatives spent the final two weeks of the campaign driving home his "negatives." Among other salvos, they accused him of trying to steal the election for the 1st District seat, which became vacant earlier this year when Democrat Sam B. Hall resigned to become a federal judge.

GOP polls taken Thursday, the basis for Republicans' optimism about the election, indicated that the plan had worked: For every voter who had a positive impression of Chapman, another had a negative impression. Hargett's positive/negative ratio remained a comfortable 3 to 1.

Chapman, 40, weathered the personal attacks by spending the closing days of the campaign tying his 38-year-old opponent to the "national Republicans," a winning strategy in a district suspicious of all institutional power, whatever its ideological flavor, and one that has not sent a Republican to Congress since 1869.

"Our polls showed the number of voters who didn't know Hargett was a Republican dropped from 30 percent to 10 percent between the first election June 29 and the runoff," Coelho said. "Once we'd done that, I knew we were in."

Coelho added that the campaign illustrated the power of the trade issue, which Chapman hammered at in the final weeks. The district has a 9.4 perent unemployment rate, and many recent layoffs have been trade-related. "You'll be hearing a lot of that in 1986," he promised.

An even more welcome tonic for the Democrats, particularly in Texas, was the simple act of breaking the GOP momentum from 1984 that has turned dozens of Democratic notables into party-switchers.

"This sends a powerful message to conservative Democrats," Chapman said at his victory party Saturday night. "There's room for you in our party."

For the losers, the defeat was not all bitter. Sen. Phil Gramm (R-Tex.), a former Democrat who, more than anyone, turned the special election into a test of realignment, argued persuasively that a Republican getting 49 percent in "yellow-dog Democrat" country is a sign that change is afoot. (The term "yellow dog" is a Deep South expression describing Democrats so partisan it is said they would vote for such an animal before turning to a Republican.)

Gramm also took solace from the makeup of the crowd at Hargett's election headquarters Saturday night at an American Legion Hall in tiny Linden.

"Look around," he said, pointing to a roomful of farmers and others in cowboy hats and sneakers, and a back lawn crowded with pickup trucks. "Does this look like a Republican gathering?"