There is no leading contender in the race to acquire Adam Opel GmbH, General Motors Co. says.

Belgian private-equity firm RHJ International and a consortium led by Canadian parts supplier Magna International Inc. are locked in a struggle to claim Germany-based Opel, a deal which includes the British brand Vauxhall.

“Despite media reports to the contrary, GM has NOT specified its preference for a bidder,” GM Group Vice President John Smith says today in an Internet blog. “We remain fully open to reaching a satisfactory conclusion with either bidder.”

The Magna bid is “clearly preferred” by politicians and organized labor in Germany, Smith says. But the supplier’s overture featured intellectual property contingencies “that simply could not be implemented.”

Industry observers have suggested GM could be setting itself up for a fall if it continued to share platform and powertrain technologies with Opel’s buyer.

“GM has partners in other parts of the world who have joint ownership of these assets,” Smith says, adding the auto maker remains in talks with Magna to “resolve these challenges.”

Twice since July 8, Magna reportedly has upped its offer for Opel. The supplier confirmed it has made a pitch for a 27.5% stake in Opel, an increase from 20%.

Magna partner, Russian lender OAO Sberbank, also would acquire 27.5%, giving the consortium a controlling interest, compared with GM’s retention of a 35% stake and a 10% share owned by Opel employees.

In addition, Magna has sweetened its proposed investment to include a E350 ($496 million) up-front cash payment, followed at a later date by E150 million ($212 million) in convertible bonds.

Previously, Magna offered E100 million ($142 million) in cash and E400 million ($567 million) in convertible bonds.

However, the bid from RHJI is complete and “would represent a much simpler structure and would be easier to implement,” Smith concedes.

“Our desire and intent throughout this very long and difficult process is to create the best scenario for the future of Opel/Vauxhall,” he adds.

“It’s our hope that we will be able to present a very clean and workable bid for the consideration of the Opel Trust Board and German authorities that will be selected on the best economic and societal analysis.”

Smith does not mention Beijing Automotive Industry Holding Corp., which also had submitted a bid.