The California legislature is poised to grant political representation to illegal aliens in the 1981 legislative reapportionment, and then use the immigrants in a bid for more congressional seats in the 1982 elections and a larger number of presidential electors in 1984.

The illegal aliens, who are believed to be swelling the state's population by hundreds of thousands annually, also could be an important factor in California's claims to a larger chunk of revenue-sharing and other federal funds distributed on the basis of census statistics.

To ensure the fullest possible count of illegal aliens in California, as well as citizen minorities who previously have been undercounted, Gov. Edmund G. (Jerry) Brown Jr. and the Democratic-controlled legislature have allocated $800,000 in the current state budget to promote participation in the census by these groups. Also, the Census Bureau has agreed to devote a portion of its billion-dollar budget to the same purpose: a public relations campaign in minority communities.

The promotional effort, already under way, is aimed mainly at allaying the fear of illegal aliens and welfare recipients who suspect that the census information may be used against them.

The net effort on California and other states will not be known untio after the 1980 census, because there is little reliable data available now on the numbers and locations of the illegal aliens and considerable uncertainty over how many will be reached by the census takers - or want to be.

However, states with relatively few illegal aliens clearly will be the losers in the final redistribution of political representation and federal money.

This new twist in the approach to illegal immigration appears to have caught many political leaders here by surprise, particularly conservatives who oppose bringing illegal aliens into the state's political process.

The issue arises from a feature of the federal census and political reapportionment process that is not commonly understood - that illegal aliens count as much as citizens.

By being counted in the national decennial census next year, illegal immigrants, in effect, could wind up being represented in Congress and many state legislatures.

In most cases, representation in the House of Representatives and the state legislatures is based solely on federal census figures. It takes roughly 500,000 people to form a congressional district.

After the federal head count next year, the states must then redraw their political boundaries, using population statistics that lump citizens and aliens together in one total.

As a result, illegal aliens will gain equal weight with U.S. citizens in the process of setting up voting districts.

Additionally, the number of presidential electors a state is entitled to is based on the size of its congressional delegation. So the more members a state can send to Congress, the more weight it will have in electing a president.

Census data collected on the immigrants next year also will help increase several states' share in many federal programs that now cost more than $50 billion annually.

In addition to revenue sharing, the approximately 100 census-based programs range from school and housing aid to community improvement projects and affirmative-action goals.

In the past, when the states reapportioned their districts, no one worried about the effect of including non-citizens. There weren't enough to make a significant impact.

But many politicians believe that the rapid emergence of a large illegal population since the last federal census in 1970 will raise the question of alien political rights as a serious issue of public policy.

"It's a whole new ball game now," said Mike McGehee, a legislative aide assigned by the state GOP leadership to research the legal and political aspects of alien rights. "And the stakes have become very high."