SAN FRANCISCO (MarketWatch) -- General Motors Corp. shares fell as much as 18% on Thursday after the troubled automaker and its auditor stoked more doubts that the company can keep its assembly lines running amid a historic dearth of new-car buyers.

“ 'Given the current administration, what are the odds that the Democrats tell the UAW to dissolve the union? It's just not very likely.' ”

The stock closed down 34 cents at $1.86 as part of a broader market pullback that brought the Dow Jones Industrial Average DJIA, -0.47% down 281 points, or 4.1%. See Market Snapshot.

Prompting the retreat, GM GM, -2.37% reiterated in a Securities and Exchange Commission filing that bankruptcy is a possibility if its viability plan, as submitted to the Congress, doesn't succeed.

The Detroit-based giant is seeking up to $30 billion in loans from the U.S. government, as well as loans from foreign governments including Canada, Germany, the U.K., Sweden and Thailand for up to $6 billion more.

Vehicle sales, which have dropped 40% in the U.S. from their peak and last month touched levels not seen in almost 30 years, will need to show signs of recovery next year under the plan.

GM's survival also depends on its ability to obtain liquidity and financing to establish an appropriate level of debt, cut costs and have consumers convinced of its viability -- as well as partly owned GMAC's ability to obtain funding from both wholesale and retail financing.

Deloitte & Touche made a similar analysis, concluding that GM's "recurring losses from operations, stockholders' deficit and inability to generate sufficient cash flow to meet its obligations and sustain its operations raise substantial doubt about its ability to continue as a going concern."

The stock market shares those fears: GM's shares have lost 90% over the last 12 months.

GM said the conclusions reached by its own auditors weren't surprising, given prior public statements made by GM regarding its liquidity position.

"The auditor's opinion has no impact on the aggressive actions we are taking to restructure our business for long-term viability." GM said in a statement. "Once global automotive sales recover and GM's restructuring actions generate the anticipated savings and benefits, the company is expected to again be able to fund its own operating requirements."

Despite the heightened concerns, Erich Merkle, an industry analyst in Grand Rapids, Mich., said he still believes that the bankruptcy route would be too messy, too complex and far too damaging to the economy.

"The best course of action, if such a thing exists, would be for the bondholders and the United Auto Workers to come to some sort of agreement to do what it takes to make GM viable," he said. "Given the current administration, what are the odds that the Democrats tell the UAW to dissolve the union? It's just not very likely."