BONN, West Germany (AP) _ Chancellor Helmut Kohl fired the manager of his conservative Christian Democratic Union in an apparent bid to bolster the sliding fortunes of the party before the national election.

Heiner Geissler told reporters at a news conference late Monday that Kohl told him he would not be renominated at the party's national conference, which begins Sept. 10 in Bremen.

''I regret this,'' said Geissler, who has been party general secretary for 12 years. ''I think it sends the wrong political signal.''

Kohl planned a news conference today to discuss party issues as well as the government's plans for responding to a flood of refugees from East Germany.

Like Kohl's reorganization of his 19-member Cabinet in April, the party shakeup appears aimed at winning back voters who have been defecting to other parties the past two years.

The Christian Democrats fared poorly in the European Parliament elections in June, and it lost control of the Berlin and Frankfurt city governments in other elections this year.

Although West Germany's economic performance has continued strong, voters have become increasingly disenchanted with the Bonn government's policies toward sharing of NATO defense burdens and the steady stream of refugees and asylum seekers entering the country.

That dissatisfaction has sent many traditional Christian Democrat voters to the political fringes, boosting support for both the ultra-rightist Republicans and leftist parties like the Greens.

Public opinion polls conducted over the past few months have repeatedly shown the Christian Democrats running a few points behind the left-leaning Social Democrats in popularity.

The liberal Free Democratic Party, currently aligned to Kohl's conservative-led coalition, has also hinted recently that it may switch allegiances before the next national elections in December 1990 if the party continues its slide at the polls.