General Motors Corp. (GM) bondholders mounted a lobbying campaign on Capitol Hill Wednesday to oppose a federal government-backed plan that would wipe out much of the money they are owed.

More than 20 GM investors planned to meet with their representatives in Congress Wednesday and Thursday, contending they aren't getting a fair shake in negotiations over the restructuring of the nation's largest auto maker. The group, claiming to represent hundreds of individual GM investors from around the country, has held rallies in Michigan, Pennsylvania and Florida.

Calling themselves Main Street bondholders, the investors said they are "average" Americans who purchased GM bonds "as a part of their financial planning for retirement, medical expenses, small business expenses, and providing for their children's education," according to a statement. The meetings in Congress, the group said, are intended to highlight their "interest in a fair and equitable solution to GM's financial crisis."

Under a plan backed by the U.S. Treasury Department's auto task force, GM is offering unsecured bondholders a 10% equity stake in the company in exchange for $27 billion in debt. Treasury officials have said deep sacrifices from all stakeholders are needed to ensure the company's long-term viability. The bondholders deal has met resistance.

Jim Graves, a 58-year-old software developer from Celebration, Fla., said that under those terms, he and his mother stand to lose most of the $100,000 in GM debt they have been counting on for retirement.

He pointed out that compared with the unsecured bondholders, other stakeholders, including the U.S. government, would receive bigger equity stakes in exchange for smaller amounts of debt as part of the GM restructuring. "It's stunningly unfair," Graves said.

Graves said he and other investors met with staffers of Sen. Tom Coburn, R-Okla., and Sen. Joseph Lieberman, I-Conn., who were "sympathetic." There's no consensus among investors about what would constitute a fair deal, Graves said, but he emphasized that the main goal is that they are adequately represented in the negotiations.

"We have not been able to plead our case with anyone and we've basically had 30 days to respond to this horrific offer," he said.

The bondholders group has called for a meeting with the task force. A Treasury spokeswoman didn't immediately respond to a request for comment Wednesday.

GM spokesman Greg Martin declined to comment on talks with bondholders, but said, "It was very clear from the task force that we needed to take more aggressive actions to restructure operations and reduce liabilities and debt on our balance sheet."

GM is hoping to complete deals with stakeholders by a government-imposed June 1 deadline to avoid a bankruptcy filing.

-By Josh Mitchell, Dow Jones Newswires; 202-862-6637; joshua.mitchell@dowjones.com