Greenlight manager David Einhorn told Reuters on Wednesday the automaker has refused to allow him to present his proposal to credit agencies for a formal review of how the plan would affect the company’s credit risk.

“We think the credit rating process has been unfairly manipulated,” Einhorn said. “We call on GM to allow us to work directly with the credit rating agencies.”

GM fired back that any suggestion the automaker had failed to share Greenlight’s proposal in full with the ratings agencies was “baseless and irresponsible.”

Einhorn went public with a proposal in late March that the U.S. automaker create one class of stock that pays a dividend and one that does not, but would be tied to GM’s potential growth.

The move would lower the company’s cost of capital, improve financial flexibility and boost market capitalization by as much as $38 billion, Einhorn said.

In the interview, Einhorn said his plan would also allow GM to preserve more of its cash reserves, at a time when some analysts are concerned the U.S. auto industry is heading into a cyclical slowdown.

“Our proposal is an elegant way to unlock that kind of value without drawing on the cash resources of the company,” Einhorn said.

GM has rebuffed Einhorn’s proposal, saying it would not help the automaker “sell more cars, drive higher profitability, or generate greater cash flow.” Both Moody’s and Standard & Poor’s have declared such a structure could hurt its credit rating. The views of the rating agencies are a key element in the proxy fight that will play out over the next several weeks, unless GM and Einhorn can reach a settlement.

Greenlight Capital has owned GM shares on and off for five years and now has a 4.9 percent stake, including options.

GM stock has performed poorly versus its peers and compared with the broader market. GM traded on Wednesday at just above $33 a share – the price at which GM shares went public in 2010 after the company’s government-led bankruptcy.

Dispute intensifies

The disagreement became a full-fledged proxy fight on Wednesday as Greenlight said GM’s advisers had no interest “in performing an objective analysis” of Einhorn’s proposal.

In a regulatory filing, Greenlight said GM had then “substantially altered” the term sheet for its proposal before handing it to the ratings agencies and that this different version included “various misrepresentations” of the plan.

GM said in a statement: “The rating agencies’ public statements issued regarding the Greenlight proposal clearly indicate that they understood the idea in all its facets, and would represent a credit negative if implemented. Any suggestion to the contrary is baseless and irresponsible.”

Greenlight on Wednesday nominated former AT&T Broadband Chief Executive Officer Leo Hindery, longtime Greenlight research director Vinit Sethi and Consol Energy Inc Chairman William Thorndike to the automaker’s board of directors.

Einhorn himself was not on the proposed slate.

Sethi was involved in talks with GM on Greenlight’s proposal going back to last September.

Einhorn’s fund has returned an annualized 16 percent since the firm’s launch in 1996 but its gains have been more muted recently with an increase of only 1.5 percent in the first three months of 2017.

Einhorn’s fight with GM marks the first time since taking on Apple Inc in 2013 that the closely watched fund manager has pressed management publicly to make changes, taking a page from the playbooks of so-called activist hedge funds like Jana Partners and Pershing Square Capital Management.

GM has not yet published a date for its annual shareholder’s meeting, but in previous years it has usually been held in the first half of June.

Shares of GM were down 0.2 percent at $33.84 at midday on Wednesday.

A majority of investors surveyed by Evercore ISI just after Einhorn disclosed his proposal said it would not raise GM’s value.

Greenlight’s filing comes just two days after luxury electric car maker Tesla Inc surpassed GM to become the highest-valued U.S. automaker despite selling around 76,000 vehicles last year to GM’s 10 million.