A coalition of homebuilders from the Midwest and the Northeast is heading to Washington early next month to lobby for a tax credit of up to $5,000 for buyers of new houses, an action the builders said is needed to help reduce a nationwide inventory of 400,000 unsold units.

The homebuilding industry is reeling under the effects of record mortgage rates which have suppressed home buying and have driven a number of builders out of business or into remodeling and rehabilitation.

Saying they wanted to work for "economic parity" for their areas but are not seeking government handouts, builders from the 14 states in the coalition maintain that the housing tax credit would stimulate employment in their areas.They estimated that about half of the home construction labor force currently is unemployed.

"We're trying to prevent the Northeast from becoming the Appalachia of the '80s," said Dennis M. Penman of Buffalo. He said he hopes that the 125,000-member National Association of Home Builders, which is meeting here, will make a drive for the tax credit one of its "highest priorities."

Among the states in the coalition is West Virginia, and David Smith, a Maryland builder who is a vice president of the NAHB for Maryland and Pennsylvania, endorsed the campaign.

The builders also today called for creating tax incentives for savers to help build up mortgage-lending funds in thrift institutions, which they said would help lower monthly mortgage payments as well. Interest in creating alternative savings instruments through tax incentives has run high at this convention, which is being attended by about 50,000 builders, lenders, product suppliers and others.

John Dalton, chairman of the Federal Home Loan Bank Board, told the builders that the Federal Home Loan Mortgage Corp.'s proposed program for purchasing adjustable-rate mortgages could boost significantly the amount of money being recycled into savings and loan associations for mortgage lending.

Dalton refused to sound the death knell for the fixed-rate mortgage, however.

"If we can cure inflation, then we will get back to it," he told reporters. "I'd be the last to say it is dead. . . People will still want it and will go anywhere they can find it."

Whether lenders ever will be willing to make long-term, fixed-rate loans in an era of ongoing inflation is questionable, other speakers maintained.