Warning of a "transportation calamity comparable in magnitude to Penn Central," the nation's intercity bus companies have launched a campaign to win government aid.

Industry spokesman Charles A. Webb, president of the National Association of Motor Bus Owners, has told Congress that while some bus firms "have grave misgivings" about U.S. aid, "it is now all too clear that we cannot continue to be a viable industry without federal financial assistance."

In testimony before a House Transportation Subcommittee, Webb attributed his industry's deteriorating financial condition to the "heavily subsidized competition" of competing passenger carriers, the impact of inflation on low-income people and "dramatic increases in the cost of insurance."

The nation's beleaguered bus lines have experienced a decline in ridership - as well as profits - in the past decade. Industry statistics reveal that the number of fares fell from 401 million in 1970 to 311 million in 1976.

Webb asked the subcommittee to consider a proposed bus revitalization act. This draft legislation would authorize capital grants for the construction and improvement of bus terminals, operating subsidies for essential intercity services, exemption of intercity buses from federal highway use charges and additional tax credits for investment in new buses.

Although Webb attached no price tag to the industry's proposal, the chairman of Continental Trailways, Fred G. Currey, has proposed a refundable tax credit program that would provide a 15 per cent rate of return on investment.

Sen. Russell D. Long (D-La), chairman of the Surface Transportation subcommittee, recently asked President Carter to consider a tax credit plan for the bus industry.

Long told Carter that if one per cent of the citizens using automobiles for intercity trips could be converted to intercity buses, about 208 million gallons of fuel could be saved annually.

Long's program would generate $100 million of tax credits for the bus industry each year.

"This amount compares favorably to the potential savings in motor fuel that might be achieved by other government efforts," Long wrote to Carter on May 20.

Another leader of the bus industry, Greyhound Lines chief executive James L. Kerringan, has been spear-heading an industry attack on Amtrak.

"I will cease and desist my campaign if the Amtrak board will today establish a policy of having its passengers pay at least 75 per cent of the cost of their tickets, Kerringan said in a recent open letter to Amtrak.

He said the nation should act to take Amtrak "off the welfare system and into the transportation system."

Kerringan complained that taxpayers now pay an average of 62 per cent of the cost of Amtrak tickets, a subsidy he said amounts to $25.50 per rider. His letter to Amtrak was in response to another letter from Amtrak's president, Paul Reistrup, who complained about Kerrigan's attacks on the National Rail Corporation.

The Greyhound chief noted that intercity bus companies paid over $47 million in highway use funds in 1976. He charged that Amtrak is jeopardizing the survival of the country's 450 intercity bus companies.

"We are privately owned, non-subsidized, taxpaying entities serving 355 million people in 15,000 communities most of which are not even served by Amtrak," Kerrigan said. "And the irony is that we are rapidly becoming an endangered species!"

Currey, of Trailways, recently testified that buses offer the only way to travel for a substantial portion of the country. He said 70 per cent of intercity bus travelers transportation. More than 43 per cent of bus users earn less than $10,000 a year, Currey said.

The Trailways official said bus companies could win riders away from automobiles with low fares, improved and new terminals or new and modern equipment.

Leaders of the bus industry also have condemned the Justice Department and the Council on Wage and Price Stability for their opposition to higher intercity bus fares.

According to the National Bus Traffic Association, average bus fares in 1976 were 16 per cent above those in 1974. This contrasts with claims by the government agencies that bus fares have increased 40 per cent.

The bus owners said revenues are far below levels needed to replace, maintain and modernize equipment. They supported a 13 per cent increase ini passenger fares and express rates.