A political battle over whether illegal aliens should be counted in the reapportioning of congressional districts and state legislatures is threatening to delay the 1980 census.

A suit filed Dec. 5 in U.S. District Court here would require the Census Bureau to exclude the estimated 8 million illegal immigrants from the population base used to readjust political boundaries each decade.

If the suit succeeds, six states with large immigrant populations would lose congressional seats, and 13 states -- including Pennsylvania, West Virginia and North Carolina -- would gain. Virginia and Maryland wouldn't be affected.

Rep. Robert Garcia (D-N.Y.) said yesterday his subcommittee on census and population will hold a hearing within a few weeks to discuss the issue. The suit was filed by the Federation for American Immigration Reform (FAIR) and five congressmen.

The case raises major constitutional and philosophical questions about the role of illegal aliens in American society. They are not allowed to vote -- but should their growing numbers be a major factor in the nation's political geography?

"These people have no right to be here, and yet they are being counted the same as if they were citizens," said Barnaby Zall, of FAIR, a year-old Washington-based group which lobbies for immigration restrictions.

Zall said that counting illegal aliens for reapportionment "violates the constitutional principle of one man, one vote. People who live in districts with a lot of illegal aliens will have their votes enhanced at the expense of those who don't."

The suit comes at a time when the Census Bureau is preparing for an unprecedented attempt to count Hispanic residents, both illegal and legal. Black and Hispanic groups have charged that the 1970 census undercounted minorities by several million, thus diminishing their share of federal funds, which are often distributed on the basis of census data.

Census day is April 1. FAIR has asked for a preliminary injunction to stop the census unless the government provides new forms to identify illegal aliens and takes steps to exclude them from the count used for reapportionment.

Theodore Clemence, chief of ther Census Bureau's office of program and policy development, declined to discuss the substance of the suit. However, he said, "We're concerned. It could set us back considerably. We've been planning this census for five years. We have 60 to 70 million questionnaires to deliver and have returned. We are already hiring hundreds of people a week for 400 district offices."

Census forms are already printed. The short form, to be mailed to about 80 percent of households, contains no question on nationality. The long form, to be mailed to about 20 percent, asks citizenship, but makes no distinction between legal and illegal aliens.

Clemence believes the Constitution "is clear about who should be counted. Ever since 1790, aliens have been included in the apportionment figures." The Constitution, he said, specified that indentured servants should be counted, Indians not taxes should be excluded, and slaves should be counted as 3/5ths -- a provision repealed in 1868.

While, in theory, the suit could affect the $50 billion in federal funds distributed through formulas that rely on census data, both Clemence and Zall noted that the suit addresses only the issue of political representation. Formulas set by Congress for revenue sharing, education, economic development and other programs would most likely be based on total population as well as other factors, they suggested.