SAN FRANCISCO (MarketWatch) -- The head of the Senate's government affairs committee Wednesday unveiled a series of restrictive proposals aimed at financial speculators in commodities, including one that would place an outright ban on big pension funds buying agricultural and energy futures.

The three legislative ideas from Connecticut's Joe Lieberman, which the independent senator plans to discuss at a hearing June 24, count as the most drastic efforts yet from lawmakers targeting potential culprits behind high oil and grain prices.

The most severe would prohibit private and public pension funds with more than $500 million in assets from investing in agricultural and energy commodities traded on a U.S. futures exchange, foreign exchange or over the counter, according to materials provided by Lieberman's office.

A second plan would direct the Commodities Futures Trading Commission to establish total limits on the share of the commodity market held by financial investors.

A third proposal would direct the futures regulator to impose speculative-position limits on any stakes not related to real hedging activities, an action that could limit the commodities-swaps activities of big investment banks such as Goldman Sachs Group GS, -1.14% and Morgan Stanley MS, -2.35% .

"We are not, as some continue to argue, witnessing the ebb and flow of natural market forces at work. We are instead seeing excessive market speculation at work and that is why our government must step in with new laws to protect our economy and our consumers," said Lieberman in a statement.

Lieberman will most likely introduce legislation with Sen. Susan Collins, R-Maine, after the July 4 holiday recess, said a staff representative of Lieberman. That legislation could incorporate some or all of these proposals, depending on feedback from witnesses at the hearing, as well as the public.

Investment banks and pension funds aren't waiting for that forum to make their anxiety about Lieberman's proposals known. A statement penned by six influential trade groups, including the Securities Industry and Financial Markets Association, the Financial Services Roundtable and the Investment Company Institute, warned that efforts to bar financial investors from commodities markets could "substantially harm the ability of Americans to protect themselves against inflation."

"Blaming speculation or any specific trading practice for rising or falling commodity and energy prices without real evidence of wrongdoing is misguided," said the statement. "Those kinds of charges create the very real possibility that speculators will choose to abandon these markets and use their investment dollars elsewhere," and such an exodus threatens the healthy functioning of the markets, it said.

The CFTC, under pressure from Congress to show it is working to rein in excess speculation in commodities markets, said Tuesday that it was closing the "London loophole" by making London traders of the benchmark U.S. oil contract follow the same speculative-position limits as their U.S. counterparts. See earlier story.

Despite analysis from the futures watchdog that there's little correlation between high commodities prices and the recent wave of financial investing, many big users of energy and grain markets argue that supply and demand can't be the only reason for the commodities boom. They've called for more regulation of financial investments in commodities.