Hearings will begin in the Senate this week on John Glenn's bill to create a Department of Veterans Affairs. They shouldn't take long. The proposal has overwhelming support, including the support of a president who once promised to oppose expansion of the bureaucracy.

As it happens, a good case can be made in support of the move. At present, the Veterans Administration operates under the thumb of the Office of Management and Budget. Administrator Thomas K. Turnage cannot even testify before Congress without OMB's consent.

The VA is too big an outfit for that kind of treatment. The agency employs more than 240,000 people. Its $27 billion annual budget is exceeded only by the budgets for defense and agriculture. The agency maintains 172 hospitals and 111 national cemeteries. Its life insurance program provides $213 billion in coverage for 7.2 million veterans and service personnel. Educational benefits are subsiding, but other programs are large and getting larger.

At the same time the House passed its bill to raise the VA to departmental status, the House also passed a bill to expand the present program of loan guarantees. When the program began in 1944, the government guaranteed a maximum of $2,000 on a veteran's home loan. By 1980 this had grown to $27,500. Under the House bill, the maximum would be further increased to $36,000. Only about 3 percent of veterans' loans are in default, but these involve the VA in the foreclosure and sale of hundreds of houses.

These considerations of magnitude might not be compelling, but there is one thing more. Our nation counts 27 million living veterans. Together with spouses and dependents, they constitute an interest group of more than 80 million persons. And they all vote. These facts of political life were eloquently evident in the House on Nov. 17. The House passed its version of the departmental bill by a vote of 399 to 17. Not one word was said in opposition on the floor.

The pending bills provide an opportunity -- though it is not likely to be seized -- to explore some sensitive questions: Where are we headed in terms of ''veterans' benefits''? Granted that the country owes a debt to those who have served in the armed forces, how large is that debt? How should it be paid?

These are hard questions; they will not yield to easy answers. Certainly the government owes lifetime medical care for service-connected disabilities. This is now provided: 96 percent of the VA's expenditures for health care go to patients thus disabled (or to veterans with incomes of less than $15,000). Historic precedent supports pensions to needy veterans. Ninety years after the Spanish-American War, we still pay $15 million a year to needy dependents of Manila Bay and San Juan Hill. Through educational subsidies and vocational training programs, thousands of veterans have benefited significantly. No reasoned objection can be taken to an insurance program that is self-supporting.

The thought that troubles many Americans is that at some point, and under certain circumstances, a line should be drawn on veterans' programs. More than 10 million veterans of World War II, with an average age of 64.2, are coming into sharper focus. Many of them saw combat; many more did not. Many served for only a few months in 1944 or 1945, never heard a shot fired in anger and suffered no service-connected disabilities. What debt is owed them today? Why should their hospital bills be specially subsidized?

The same questions ought to be asked about the program of home-loan guarantees. At what point does a veteran put aside his uniform and reenter the mainstream of American life? Veterans are entitled to bonus points on certain civil service examinations. Because most veterans are male, female job applicants clearly are disadvantaged. Is this special preference a continuing obligation?

The government's obligation to those who serve in the armed forces is to be fair -- even to be generous. The debt is especially owed to those who were drafted for military service. There is no way that federal pay scales can be equated between civilian and military jobs. The sailor on a North Sea frigate gets no time and a half after 40 hours a week. The pencil pusher in a Washington office is not exposed to the hardships of an Air Force station in Alaska.

All I am suggesting -- and I am not suggesting it very clearly -- is that benefits to veterans must have some reasonable limit. If the effect of departmental status is to promote unreasonable benefits, we will regret it by and by.