Early on a cold January morning more than a year ago, a police officer in Pekin, Ill., stopped a pickup truck he had clocked at 43 mph in a 30 mph zone. According to the police account, the driver briefly stopped, then resumed driving to his home a few blocks away.

Confronted in his driveway by several officers, the police say, the motorist tried to use his position in state government to escape a traffic ticket. He was eventually handcuffed and charged with speeding, failing to yield to police and resisting arrest.

What seemed to be a fairly routine traffic altercation set off a chain of events that led today to an extraordinary gathering in the ornate state Capitol here. A special 10-member committee of the Illinois House held its first meeting to investigate whether the driver, James D. Heiple, should be removed as chief justice of the Illinois Supreme Court. Only two Illinois judges, the last in the 1840s, have been impeached by the House, but neither was removed from office by the necessary two-thirds Senate vote.

The rare nature of the proceeding was underscored by the committee's attempt to sort out what constitutes an impeachable offense. The modern Illinois constitution, adopted in 1970, does not define impeachable offenses.

For more than an hour, with a retired federal judge and a former Illinois governor gently sparring over ground rules, the committee heard testimony from an array of legal experts. The one common theme they struck was that judges cannot be impeached because of controversial rulings or because they are unpopular.

"Being unpopular goes with the territory of being a judge," said Donald Hubert, president of the Chicago Bar Association.

The acerbic and outspoken Heiple, 63, has become increasingly unpopular since his election in 1990 to a 10-year term on the state's highest court. He faces a "retention election," a referendum on whether he should remain on the court, in 2000. He wrote one of the court's most controversial recent decisions, a unanimous 1994 ruling that removed a 3-year-old boy known as "Baby Richard" from the home of his adoptive parents and awarded custody to his biological father.

The "Baby Richard" case led to a public exchange of insults between Heiple and his fellow Republican, Illinois Gov. Jim Edgar, and an anti-Heiple campaign that is still being waged by Chicago Tribune columnist Bob Greene.

In this atmosphere, Heiple appears to have few public admirers, but he does have some public defenders. One of them, Chicago Sun-Times columnist Steve Neal, recently described Heiple as "an unpleasant person" who has made mistakes. "Heiple is no prize," Neal wrote, "but being a jerk isn't grounds for impeachment."

Heiple was fined $200 after pleading guilty to two petty offenses in exchange for dropping of the resisting arrest charge in Pekin. But the case began to escalate into a constitutional confrontation in January when the state Judicial Inquiry Board, which investigates charges against judges, filed a complaint against Heiple with the Illinois Courts Commission, which has power to discipline or remove judges. The complaint cited three other alleged instances in which Heiple tried to use his Supreme Court credentials to avoid traffic tickets.

The chairman of the courts commission, Supreme Court Justice Moses W. Harrison II, was named to that post by the high court only days before the Heiple complaint was filed, leading to new charges that the chief justice was trying to use a "personal friend" to manipulate the commission's ruling. Heiple's defenders retorted that the state constitution requires the commission to be headed by a Supreme Court justice, and that Harrison is no more a personal friend of Heiple than any of the other justices.

At a hearing this month, the courts commission refused to take any testimony after Heiple said he did not contest the facts of the traffic incidents. Two weeks later, acting on a resolution by a Republican legislator who said she was moved by the angry reaction to these events on a Springfield radio station, the Illinois House voted 113 to 0 to establish the special investigative committee.

Most legal scholars initially dismissed Heiple's conduct as inappropriate but far from grounds for impeachment. But the case has taken on a life of its own. A retired federal judge, Frank J. McGarr, sat with committee members today as their special counsel. Heiple was represented by former Republican governor James R. Thompson.

Thompson, the longest-serving and arguably most popular governor in Illinois history, showed he was playing for keeps. He arrived at the Capitol today with six other lawyers from the major Chicago law firm that he heads. He also filed a 20-page brief arguing that impeachment should be reserved for treason, bribery and other "high crimes and high misdemeanors," not attempts to evade traffic citations.

Committee members appeared grave as they listened to explanations of what constitutes an impeachable offense. "We understand the seriousness of the challenge," said Rep. Barbara Flynn Currie (D), the panel's co-chairman. CAPTION: James D. Heiple, chief of Illinois Supreme Court, is represented by former governor James Thompson.