Shifting political tides on Capitol Hill are threatening to engulf the Chicago futures markets in new federal controls that could cost them much of their long-cherished, free-wheeling independence.

For more than a year, the Chicago markets, aided by powerful supporters in Congress, have been able to fight off a Bush administration plan that called for sweeping changes in the way the government regulates the futures industry.

But the future industry's victory, it appears, could be short-lived.

A new drive to rein in the Chicago markets is being led by Sen. Patrick J. Leahy (D-Vt.), chairman of the Senate Agriculture Committee, which once offered only rock-solid support for the Chicago futures markets.

Leahy and a group of key senators from the Senate Banking and Agriculture committees are pushing for passage of a compromise that would be less restrictive than the administration plan -- but would curtail the freedom enjoyed by the Chicago markets.

Leahy said he intended to move quickly and vigorously for passage and believed that the support in the two committees will give the plan "a good chance of success on the floor." Chances for passage in the House are less clear, sources said.

One of Leahy's motivations is that he has been trying unsuccessfully for a year to win passage for a bill that renews the funding of the Commodity Futures Trading Commission (CFTC), which oversees Chicago's markets. But it has been blocked in the Senate by the debate over the administration bill.

Perhaps more importantly, the CFTC funding measure contains tough new trading rules for the Chicago markets, adopted by the Leahy committee after trading scandals in that city. Leahy wants those new standards, which include enhanced tracking of trades and steeper fines for rule violations, adopted as soon as possible.

Federal Reserve Chairman Alan Greenspan helped quicken the chances for change yesterday when he told a Senate Agriculture Committee hearing that if Congress insisted, the Fed would accept the job of regulating margins on stock index futures in Chicago.

Margins are the deposits that investors must put down in order to trade in the Chicago markets. Futures contracts on stock indexes, or baskets of stocks, are a bet on whether the prices of the stocks in New York will rise or fall by a certain future date.

At present, the Chicago markets set their own margins, a practice that has become highly controversial -- but it has been staunchly defended by the Chicago exchanges.

In the past, the Chicago markets have been accused by the Securities and Exchange Commission of encouraging speculation by keeping margins low to attract more investors.

Charles M. "Chip" Seeger III, Washington counsel for the Chicago Mercantile Exchange, said yesterday that the futures markets did not want the federal government to have authority over margins.

"It would be very harmful," he said.

Greenspan said he would prefer that the job of controlling margins in Chicago go to either the CFTC or the SEC, but he said the Fed recognized "the difficulty and urgency of resolving this particular question."

In past debate, it has become clear that Congress would not give such power to the SEC. At the same time, the CFTC has said the markets should set their own rules.

The Fed already supervises margins in the stock market, delegating its day-to-day authority to the SEC. Greenspan implied that if the Fed took over supervision of futures margins, it was likely to delegate day-to-day authority to the Chicago markets.