Phil Gramm, the conservative Democrat who quit Congress last month and is seeking reelection as a Republican, says there's only one issue in Saturday's special congressional election here: himself.

But his precinct walkers put it a little differently when they knock on doors.

"Phil had to choose between us and Tip O'Neill, and he chose us," they are instructed to tell prospective voters. "Now he needs our vote."

Gramm's literature pictures House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and House Majority Leader James C. Wright Jr. (D-Tex.) and says the two "want you to stay at home Saturday," adding, "The nation is waiting to hear what you have to say."

Gramm is invoking O'Neill and Wright, who are not terribly popular here, because he wants a big victory in this hastily called election, knowing that if he does not do well his statewide aspirations could be hurt and he will become a vulnerable target for Democrats in 1984.

Gramm says he believes he can win the race outright against 10 opponents, and most Democrats privately agree. But after the big Democratic victory in Texas in November, many Democrats hope, also privately, that they will be able to deal him a surprise: hold him to less than 50 percent of the vote, which would force a runoff in April.

No one is certain just how Gramm will be affected by running as a Republican in this conservative Democratic district.

Gramm's strongest challenge comes from former state representative Dan Kubiak, who has the support of the Democratic Party organization. Also running is humorist John Henry Faulk, who was blacklisted during the Joseph McCarthy era and was urged into the race by fellow liberals, many of whom have deserted him for Kubiak.

Gramm has had a huge advantage. He quit Congress and changed parties on Jan. 5, after his Democratic colleagues threw him off the House Budget Committee for his sponsorship and active support of President Reagan's economic programs.

In one of his last acts in office, former Republican governor Bill Clements gave disorganized Democrats just 38 days to find a candidate, raise money, organize and get out the vote when he called the special election for Saturday. Democrats went to court to delay the election, but failed.

Gramm started the race with $200,000 in the bank, and has raised at least another $500,000. He said he will spend about $500,000, including at least $250,000 on television ads and another $100,000 on direct mail.

Estimates by others indicate that Gramm may end up spending as much as $700,000, which would mean the champion of a lean federal budget is going through about $13,000 to $18,000 a day.

By the time the race is over, Gramm's workers will have called about 95,000 voters trying to identify support, made another 50,000 calls to turn out Gramm voters and sent out more than 370,000 pieces of direct mail.

"I have run the best campaign I have ever run," Gramm said. "I have worked harder than I've ever worked in a a campaign. I have shaken more hands, I've walked more miles, gotten more people involved."

But, he added, "We're going to find out to what extent philosophy transcends partisanship. That's what this race is about."

After initial stumbling, Democrats have rallied behind Kubiak, who has attacked Gramm viciously. He said the former Texas A&M economics professor has "mocked the poor, deceived senior citizens, voted against farmers 90 percent of the time, and brought nothing into the district . . . . "

Kubiak has raised only about $80,000, about a third of it from labor.

Gramm, meanwhile, says this is the most liberating election he has ever run.

"It really is an election where I'm asking people to vote for me on the basis of what I believe and what I have done," he said.