BATON ROUGE, La. (AP) _ Gov.-elect Buddy Roemer is testing his influence by backing a candidate to take his old seat in the U.S. House of Representatives, but so far his favorite is running no better than second.

State Sen. Foster Campbell, the front-runner in the field of 10, says he has not worried about Roemer's endorsement of Stan Tiner, former editor of the Shreveport Journal.

''I'm just trying to see folks and I'm taking it day by day,'' said Campbell, a Democrat.

If no candidate wins a majority of the non-partisan vote on March 8, a runoff between the top two would be held on April 16.

The winner will fill Roemer's unexpired term and faces re-election this fall. Roemer resigned the seat effective March 14, the day he takes office as governor, replacing Edwin Edwards.

The field also includes state Rep. Claude ''Buddy'' Leach, who lost the seat to Roemer in 1980 after one term during which he was acquitted of charges in a vote-buying scandal. Leach later returned to the state House but lost a race last year for state treasurer.

The other candidates include Jim McCrery, the lone Republican and formerly Roemer's top aide; June Phillips, an administrator at Southern University- Shreveport and the only black in the race; Mayor Gene Byars of Minden; Shreveport lawyers Troy Bain and Marshall Jones; Charles Martin, a retired Shreveport businessman; and Mitchell E. Marsh, a fencing contractor from Mansfield.

Recent polls by the Journal and joint polls by the Shreveport Times and KSLA-TV have shown Campbell ahead. In the Journal poll published Feb. 19, Campbell had the support of 26 percent, followed by Tiner with 16 percent and McCrery and Leach close behind.

The Times-KSLA poll released on Feb. 26 had Tiner running third, five points behind Leach.

Campbell, a state senator since 1976, would not predict who would be in the runoff with him.

''Whoever I'm in a runoff with, I can beat,'' he said.

Roemer, who insists he hasn't formally endorsed anyone, recently appeared at a fundraiser for Tiner and gave him a campaign contribution.

''That's it,'' Roemer said of his efforts. ''I said publicly, the truth. 'Who am I going to vote for?' I said Stan Tiner. We're 20-year friends.

''But frankly, I've got other friends in that race. June Phillips, great person. Jim McCrery, great person. I'm leaving five people out and I shouldn't have gotten into this.''