Employes of the Interstate Commerce Commission, the nation's oldest regulatory agency, will start taking off one day a week without pay beginning April 1 if a little war between the commissioners and Sen. Mark Andrews (R-N.D.) is not settled.

At issue is a matter of $4.5 million, money that the ICC says it needs to pay its employes but that Andrews' Senate Appropriations transportation subcommittee trimmed from the ICC budget for the current fiscal year. Congress approved the cut.

However, the ICC, with the full concurrence of the Office of Management and Budget, spent the first half of the fiscal year operating as if it had full funding for its approximately 950 employes. The OMB included a $4.5 million supplemental appropriation for the agency in the new budget.

That supplemental appropriation appears to be in trouble, at least with Andrews. As a result, the ICC plans to live within its budget by cutting its employes' salaries 20 percent between now and Sept. 30, the end of the fiscal year. In the meantime, ICC travel has been curtailed and other efforts made to cut expenditures, according to Managing Director Martin E. Foley.

To avoid violating the Anti-Deficiency Act -- the law that has twice forced the Reagan administration to "close down the government" for lack of funds while Congress dithered at the start of a new fiscal year -- the ICC will have to begin furloughs April 1 if the supplemental appropriation is not moving.

The House Appropriations transportation subcommittee is scheduled to mark up the legislation Thursday. Andrews' subcommittee will hold its hearing March 28, after postponing it from March 12. A senior ICC official acknowledges that now, with the cost-cutting measures that have been taken, the agency could avoid furloughs this fiscal year with $1 million less than the supplemental request.

Andrews, who despite his political affiliation has been a frequent critic of transportation deregulation, makes no secret of his disenchantment with the approach of the current ICC, which is supposed to oversee railroads, buses and trucks. In Andrews' view, and that of many ICC critics, the agency today rubber-stamps transportation-company requests and ignores consumer concerns.

That view has led to the formation of the Coalition United for Rail Equity (CURE), in which Andrews and Sen. Russell B. Long (D-La.) are prominent. CURE seeks changes in the Staggers Rail Act of 1980, the rail-deregulation statute. The railroads oppose amending the Staggers Act.

It didn't help the ICC last April, when Chairman Reese H. Taylor Jr. testified before Andrews' subcommittee that the commissioners had not held a regularly scheduled open meeting since the late 1970s because, he said, "It's hard to get a genuine discussion" in front of "the entire ICC constituency."

Many members of Congress found that odd, one reason Andrews' budget cut sailed through conference. The ICC has since held a number of open meetings before the entire ICC constituency, where the hostility and lack of communication among its members -- particulary between Taylor and Commission Vice Chairman Heather J. Gradison -- could be seen by all.

But Taylor and Gradison are united in support of the supplemental appropriation. Taylor, who dislikes being interviewed, last week was neither available by telephone nor in person. Gradison, in a telephone interview, said if there is no supplemental appropriation, "The furlough option is really the only option."

ICC employes, reluctant to be quoted by name, do not think that a 20 percent pay cut is a good idea. "Many of my people live from day-to-day and will be severely hurt," one upper-level supervisor said.

Andrews is adopting the hard-line view that the ICC knew what its budget was and has not taken steps to meet it. "They have been spending it just as though there wasn't any cut," Andrews said. "If people are allowed to get by with that stuff, why even have a Congress?"

Andrews denied that he is punishing the agency because he doesn't like its policies. "That isn't the way you do business down here," he said. "We looked at that hearing [last April] and we found there were some flagrant problems . . . . They were not hiring people to protect the shippers, small grain elevators, whatever."

ICC records show that since the Staggers Act was passed, railroads have sought to abandon 5,000 miles of track in North Dakota, but that the ICC has permitted the abandonment of only 202.9 miles.

One of Andrews' frequent allies is Sen. Larry Pressler (R-S.D.). But not this time. Pressler, who met with Taylor Friday, said he will write a "Dear Colleague" letter supporting the supplemental appropriation. To deny it, Pressler said, "would destroy the ICC. By putting them out one day a week, you might as well close them down."