The leading candidate for a spot on the Federal Trade Commission is George Douglas, a Texas economist who once co-authored a book with FTC Chairman James C. Miller III.

However, sources say that although he is the White House's choice for the Democratic seat on the commission, he may run into trouble with Congress during his confirmation hearings as Senate Democrats question him closely about his party affiliation and an economic study he did for the commission in 1979.

Democrats are concerned that Douglas is not really a Democrat because he voted in the Texas Republican primary in 1980. In Texas, citizens do not register their party affiliation and can vote in whatever party primary they wish.

But even more significant to some Democrats is the study he did at the FTC staff's request. It was so heavily criticized by commission opponents that an agency judge refused to enter it as evidence in an FTC proceeding, according to a transcript of the hearing.

The study was commissioned to bolster the staff's arguments that Pillsbury Corp.'s acquisition of Fox Deluxe Foods was anticompetitive and illegal.

However, in defending the merger, Pillsbury lawyers attacked Douglas' study--with Douglas on the witness stand--charging that there were at least 35 errors in the study which may have changed its results.

Given this sharp criticism, the administrative law judge hearing the case, Joseph P. Dufresne, said he would not accept it into evidence unless the discrepancies were "cleared up."

Although the FTC staff then asked Douglas to fix the study, the economist never did.

Douglas was vacationing in the Bahamas and was unavailable for comment. However, according to Miller, the errors had been committed by one of Douglas' subordinates who multiplied an incorrect set of numbers. Although the errors were minor, Miller said Douglas declined to revise the study because he hadn't been paid for a large portion of his work and feared that even though he would spend more time and money redoing the study, there was a good chance he would never get paid the amount that was owed him.

Miller also said that Douglas was a registered Democrat before he moved to Texas and that the economist only voted in the Republican primary because the Democratic primary "was just a beauty contest."

If nominated, Douglas will be the second person proposed for the Democratic FTC slot. The Senate Commerce Committee refused to confirm President Reagan's first nominee, F. Keith Adkinson.