A showdown between freight intermediaries and major U.S. ship lines comes to a climax Tuesday as the Ocean Shipping Reform Act heads to the Senate floor for a make-or-break vote.

It's a battle the intermediaries are likely to lose in what would be a major victory for U.S. carriers and their big customers, mostly industrial firms and retailers.It also would be pay back for longshore labor, which has been getting stung for years by freight consolidators who take business away from union docks.

Congressional aides and industry officials say the bill is likely to sail through the Senate by unanimous consent after one remaining amendment is defeated by an expected 2-to-1 margin.

Backers of the legislation, including Sea-Land Service Inc., the largest U.S. shipping line, and the National Industrial Transportation League, the leading shippers' group, hope to orchestrate a quick turnaround in the House, which will be needed to make the bill law.

Senate leaders have squeezed out most of the remaining controversy surrounding the bill, which attempts to improve customer service options in the highly regulated field of ocean shipping contracts.

Sen. Kay Bailey Hutchison, R-Texas, the bill's lead sponsor, along with Senate Majority Leader Trent Lott, R-Miss., and maritime leader Sen. John Breaux, D-La., crafted a compromise that stops far short of the dramatic deregulation envisioned three years ago.

The bill would leave the Federal Maritime Commission relatively intact and preserves the industry's broad antitrust immunity for pricing, but eliminates the requirement to file tariffs and deregulates contracts. Ship lines would be required to publish standard rates. Carrier conferences operating under antitrust immunity still would be subject to FMC review and approval.

Pared from its more radical 1995 version, the bill is supported by a coalition of maritime interests, including big shippers, major U.S. carriers, ports and labor.

The last of the remaining issues will be aired Tuesday in an amendment offered on behalf of freight consolidators, defined as non-vessel-operating common carriers under the landmark Shipping Act of 1984.

The amendment, offered by Sen. Slade Gorton, R-Wash., is likely to be defeated, congressional aides said. A second controversial aspect of the bill will be smoothed over in carefully targeted floor statements offered by Republicans on behalf of shipper associations whose interests were championed by Sen. Thomas Harkin, D-Iowa.

Sen. Gorton's amendment would extend to NVOs a core provision letting ship lines and customers negotiate confidential transportation contracts. Without it, some NVOs fear ship lines will pick off their customers.

''I think you are going to see a lot of NVOs whose market share quickly erodes and gets chewed up,'' said Alan Baer, president of Ocean World Line, a large freight consolidator based in New York.

Specifically NVOs, while they can enter a confidential contract with a carrier to charter space on oceangoing vessels, would not then be able to go to the market and resell that space confidentially, Mr. Baer says.

Instead, he fears ship lines would be tempted to ''back sell'' the NVO's clients, whose contract rates still would be filed under existing rules kept in place by the bill. Ship lines would be able to analyze the NVO's margins and undercut the consolidator by going directly to customers with an offer of a better deal.

Other NVOS are less concerned.

Alex Knowles, chief operating officer of Direct Container Line in Long Beach, Calif., said the large NVO does not see a major problem with the bill.

''There is a market demand for our services. We foresee that demand continuing under the legislation with or without the (confidential) contracting right. We are here to stay,'' Mr. Knowles said.

The National Industrial Transportation League, which had kicked off the shipping reform initiative after Republicans won control of the House in 1994, is taking no chances when it comes to the Senate vote Tuesday.

Although there is sympathy among shippers for Sen. Gorton's amendment, the provision is vehmently opposed by ship lines and labor, who have threatened to withdraw their support for the bill if the NVOs win.

Sen. Gorton is not believed to have more than 35 votes for his amendment.

Meanwhile, the so-called Harkin language, which clarifies congressional intent on another issue involving consolidators, has been agreed to. Sen. Hutchison will make remarks clarifying that carriers cannot refuse to deal with shippers associations, a new kind of cooperative structure that is gaining influence.

With the backers of the bill confident they can win quick House approval, the pending enactment of the shipping reform act clears the congressional maritime agenda, opening the way for Senate action on a previously delayed treaty on shipbuilding.

The treaty bans government subsidies of shipyards and is a Clinton administration priority. It had been held hostage by Sen. Lott until the fate of the shipping bill was decided.

Last week, a key aide said the treaty is likely to win Senate approval this year now that a compromise on the shipping reform bill is virtually done.