The Justice Department yesterday filed suit against California, Illinois and Pennsylvania to force them to comply with the federal "motor voter" law that allows voters to register through the mail or when they apply for state driver's licenses or social services.

In a move that fuels an already heated debate between the Democratic administration and some defiant Republican governors, Attorney General Janet Reno sought federal court orders to make the three populous states implement the law. She also left open the possibility that two other resistant states, South Carolina and Michigan, would be sued as well. "This is a common sense law that is already making voting more available to all Americans," Reno said. "Congress has the authority to regulate federal elections, and it used that authority when it passed the law. We must now use the authority that Congress gave us to enforce it."

Reno and other Clinton officials argue that states cannot simply defy the 1993 National Voter Registration Act, which passed with overwhelming Democratic support and strong opposition from Republicans, who criticized the measure as a costly infringement upon states' rights.

Democrats, who believe they will likely most benefit from a surge in registration, contended that the law would simplify the process for the estimated 70 million Americans who are eligible to vote but have never registered. Republicans maintained that the law is simply another unfunded federal mandate requiring states to implement expensive measures without new funding. All three of the states that were sued have Republican governors.

A Congressional Budget Office analysis of the possible costs of implementing the law suggests it will cost less than an average of $1 million in each state annually. Officials in heavily populated Florida, for example, said yesterday they expect the statewide cost of administering the motor voter law will be about $1.1 million. Florida is averaging 3,000 new voter registrations a day through driver's licenses.

But the partisan division over the issue has intensified recently. Last month, California Gov. Pete Wilson (R) filed a federal lawsuit seeking to bar enforcement of the law, and he ordered state agencies not to implement it. Yesterday Wilson, considered a likely presidential contender in 1996, was again quick to challenge Reno and the Clinton administration.

"Is it fair for Californians to wait in longer lines to get a driver's licenses or file an application for welfare because the federal government is imposing a new unfunded responsibility on the states?" Wilson said in a statement. "I believe it is not. . . . Unfortunately, what Attorney General Reno's rhetoric fails to recognize is that California is willing to agree to implement a national voter registration program as soon as Washington comes up with the money to pay for the additional burden foisted upon state workers."

Newly elected Pennsylvania Gov. Thomas J. Ridge (R) said the Justice Department had not given his administration enough time to comply with the law and suggested the federal government was "wasting taxpayer money" in filing suit. "It's unfortunate the Justice Department chose to flex its muscles rather than working with my new administration to accomplish our shared goal of improved voter registration," Ridge said.

However, Deval L. Patrick, assistant attorney general for Justice's civil rights division, said his staff met with Pennsylvania officials Friday and "were sent packing."

Justice Department officials also noted that state officials were notified of the law's requirements last May. They said the vast majority of the states were working toward or in compliance with the law by the Jan. 1 implementation deadline.

The Illinois legislature failed to implement the motor voter law because of concerns "that this was an additional unfunded mandate that would increase the chances of election fraud," said Mike Lawrence, spokesman for Gov. Jim Edgar (R). "The governor understands those concerns. We think it is ironic that the Justice Department has sued us while there is a debate on unfunded mandates."

Although cost has been a central part of the motor voter debate, the projected costs vary widely depending on the party making the estimate. Wilson has maintained the law would cost his state $35.8 million annually to put into effect, but the state's former chief election officer, a Democrat, claims it can be done for as little as $5 million.

The Congressional Budget Office analysis completed last February estimated that the nationwide cost to states and localities would be about $20 million a year for the first five years.

The struggle between the Justice Department and these states is part of a broader movement championed by Republican leaders who hope to shift more power from the federal government to the states. Yesterday some Republican senators said the motor voter law is indicative of a free-spending federal government commandeering limited state resources.

Sen. Paul Coverdell (R-Ga.) said Reno's action was a "classic example of a bullying government placing an extreme financial burden on our states." Coverdell is sponsoring a bill that would not require states to administer the law until the federal government provides funding.

Republican Govs. John Engler of Michigan and Carroll A. Campbell Jr. and his successor, David Beasley, of South Carolina, have openly defied the act. Engler ordered state officials not to implement parts that cost money until the federal government paid for them.

Campbell vetoed compliance legislation as he left office. Beasley promised to "fight the Justice Department all the way down the line."

Justice officials say they have seen signs of cooperation from those states and will withhold action for a "short period." CAPTION: JANET RENO