Rep. John D. Dingell (D-Mich.) is one of the pit bulls of Congress, and he likes nothing better than chewing on the leg of William Reilly.

Reilly, the head of the Environmental Protection Agency, is tailor-made to whet Dingell's appetite -- a career environmentalist, driven to clean up the air. Dingell is a career politician, driven to protect his constituents, the auto makers and auto workers in Detroit.

Ever since George Bush declared himself to be the "environmental president," Dingell has had his hands full. It isn't that Bush has turned out to be a defender of the environment. It's simply that Bush stumbled upon an EPA director who is.

For years, Dingell has harped on the EPA for its lax protection of nearly every element of the environment, except the air. Dingell's favorite target has been EPA's failure to clean up hazardous waste. But when it comes to air pollution, the congressman sings a different tune.

In past years, Dingell has fought efforts to put more pollution controls on cars because it would cost the car makers big money. But when changes to the Clean Air Act were introduced in Congress, Dingell looked up from chewing on Reilly's leg and saw that public sentiment was not going his way. It was time to compromise.

Dingell met halfway with the leading pollution fighter in the House, Rep. Henry A. Waxman (D-Calif.). They worked out a compromise and passed tough revisions to the clean air laws last month.

Dingell's fellow Democrats were breathing hot on his neck. They remembered how Bush stole the "environmentalist" label from Michael S. Dukakis by focusing on Dukakis's failure to clean up Boston Harbor. The last thing the Democrats needed was one of their own standing in the way of cleaner air.

It was Dingell who caved in, but now Reilly is watching his ankles. The chilly relationship between the two men turned to ice in the clean air debate, sources told our associates Scott Sleek and Tim Warner. Dingell even confronted Reilly at a Washington party and gave him a tongue-lashing. In a speech to the American Public Power Association in January, Dingell implied that Reilly was a dilettante who didn't care if workers in Detroit lost their jobs.

During the clean air hearings, Dingell made Reilly pay for every concession he got. The congressman drilled Reilly like a prosecutor badgering the star defense witness. After Dingell had agreed to sponsor the bill, he demanded minute details about how the new emissions standards would affect car makers, he complained about the cost of cleaner fuels, and he carped about the increased power Reilly would have.

The exchange was so cold Waxman had to break the ice: "Mr. Reilly, I think you're fortunate Mr. Dingell is the lead sponsor of your legislation. Otherwise, I think he would be critical of it."

Reilly will continue to pay the price of victory. Dingell would like to blame Reilly for the EPA's failure to issue new guidelines for the industrial and military cleanup of hazardous waste dumps. But as we reported earlier, the Office and Management and Budget has derailed those guidelines.

Rather than pick on Budget Director Richard G. Darman, Dingell has nagged Reilly about the rules.

An administration source summed up the nagging: "This is just clean air politics."