In his new federal budget, the president is sending the 50 states back to their corners.

It's hardly news that Ronald Reagan favors government decentralization. It has ever been the spine of his rhetoric. But in the 1986 budget, he says more clearly than before that we are not all in it together.

His America takes us back to the days when we were separate colonies. Today, it seems, we are bound only by the Pentagon budget, the Internal Revenue Service and the Super Bowl.

One line sets the tone: "This administration is committed to the philosophy that those who benefit directly from a specific federal service should pay the cost of providing that service where charges can be reasonably applied."

This is the rationale for increasing user fees in the national parks. Maybe it's right that the people who drive campers to Yellowstone should pay more to get in. It costs only $2 now.

But people who never go to Yellowstone think that the park belongs to them and perhaps do not begrudge the campers the mountain air and the bears. They may get there themselves some day.

The underlying assumption has been that regions have different needs. New York has welfare recipients; Montana has Indians. Mississippi needs flood control; California needs water. Eastern states need cops; western states need forest rangers. The North needs snow plows; the South needs smudge pots. And somehow, while everyone grumbles, we all get approximately what we need because we're one big country and it all evens out.

But the Reagan budget proposes that the states look after their own.

The central government's business is to defend the country, collect the taxes and give private enterprise its head. Everything else is extra.

We are not entitled, for instance, to good train service. That's a luxury. Amtrak was an effort to give us classy, European-style rail service. But the Reagan budget would eliminate subsidies for Amtrak. Amtrak has not made it in the market place, and worst sin of all, "On many routes it provides unfair, subsidized competition for private-enterprise bus and airline firms."

And says the Reagan budget, "55 percent of its Northeast corridor passengers in 1983 had incomes above $30,000."

This is interesting, because the premise of the first four Reagan budgets was that the poor were gouging the government. Now Reagan has found out that the middle-class also has planted its feet in the trough.

The section on student loans confirms his new suspicion: "Twenty-five percent of all college freshman recipients of federal student aid have family incomes that place them in the upper half of the income distribution."

As a matter of fact, the middle-class may be worse than welfare cheats, the budget complains. "Pell Grant surveys document overpayments in 19 percent of all awards; far in excess of AFDC, Food Stamp and Medicare error rates."

So now the government will limit loans to $5,000 and further restrict eligibility.

It is true that some students whose families can afford to pay have taken out loans. It is also true that some student borrowers have become dreadful deadbeats: more than $4.5 billion is owed the government.

But student loans brought the American dream closer to many young people who otherwise never could have gotten a diploma and the advantages and opportunities that go with it.

The 1986 budget has not overlooked the poor or their taking ways. The Job Corps, which helps them, would be eliminated. A trainee costs too much: $15,200 a year -- which "nearly equals the cost of sending a student to Harvard or Stanford universities."

It's true. But the cost of keeping a youth in jail is almost double -- $30,000.

The president probably could do it all -- cut Medicaid, Medicare, child nutrition and AFDC; eliminate funds for Legal Services, some mass transit and Urban Development Assistance Grants.

He could, that is, if it were not for the additional $32 billion he wishes to confer on the Defense Department. His free-floating suspicions about freeloaders stop at the Pentagon's door. Others may see $7,000 coffee pots and $600 toilet-seat covers; Reagan sees rectitude and careful spending.

It fits in with his notion that the federal government is obliged only to provide arms and 1040 forms. The rest -- comfort, help, recreation -- is the states' business. As for the unfortunate, the Reagan administration philosophy was pretty well summed up by budget director David A. Stockman. Suffering farmers, he told a Senate Committee, deserve what they get.