The guessing now is that the change in the lineup of Democrats and Republicans in Congress after the November election will be hardly more than the normal losses that can be expected in an off-year election. That would mean a loss for the majority party in the neighborhood of 25 seats in the House out of the 280 they now hold. And this, let it be added, when President Carter stands at a lower rating at this point than any of his five immediate predecessors.

The reason, as representatives who talk frankly readily say, is the enormous advantage the sitting member has. The value of his perquisites today adds up to a least $500,000 a year - and that may be a conservative estimate.

Staff alone accounts for more than $250,000. This covers staff in the representative's district. Three or four staffers in the district offices are schooled to care for constituents' needs. They are nursemaid-governesses smoothing the path through the tangled forms and regulations of government. Even the congressman himself sometimes takes over details from a favored constituent.

"Sure, I get a letter complaining that the fellow's garbage isn't being picked up," one representative said. "So what do I do? I sit down and write a personal letter to the town council or maybe to an alderman who happens to be a friend of mine and I get results."

The latest extraordinary perk is the mobile office. It is a van with a staffer in charge that tours the district as a kind of traveling billboard, since, in not inconspicuous letters, it carries the congressman's name. Parked in a shopping center or near a recreation area, it is an invitation to one and all to come in and talk about troubles with their television repairman or why the Veterans Administration has failed to approve a worthy claim.

A veteran of nearly 30 years in the House, Rep. Richard Bolling (D-Mo.), who helped at the start of his career to initiate the service of staffers in the district, says the mobile office is mighty useful. In sprawling Kansas City, his bailiwick, the Bolling office is on tour four days a week. One useful service the mobile performs is to register voters, and this is particularly true with registration at a low point and the possibility that voter turnout in November may be at an unprecedented low.

Besides staffs and district offices, the incumbent gets a newsletter printed and distributed free of charge. He also has access to radio and television facilities in the House Office Building that give him an advantage with stations back home.

Hammered away at again and again by earnest reformers, the two-year term for members of the House is an anomaly that defies reason. No sooner are you elected than you must start to work for your reelection.

As the experience of Rep. John B. Anderson, (R-Ill.), illustrates, it has become even more of a costly burden. One of the ablest and most knowledgeable members of Congress, Anderson is rated liberal-to-moderate. As such he was targeted for elimination by the conservativs in the Republican primary in his home district of Rockford, Ill.

His opponent was the Rev. Donald Lyon of the Church of the Open Bible, whose political views barely postdated the Stone Age. Conservatives, mostly from outside the state, poured in money to bring the Reverend Lyon's spending total to $240,000, and that did not include the cost of the direct-mail antii-Anderson campaign conducted by the highly profitable operation of Richard Viguerie in Virginia.

Facing that kind of challenge, Anderson spent more than $160,000, and he won by 58 percent to 42 percent. Against a little-known Democratic opponent, he is considered a likely victor in November.

A recent poll showed the paradox of today's Congress. As a body Congress had a low rating, comparable to that of the president. But as individuals attuned to the wishes and needs of their constituents they rated fairly high. It is the role of nurse ready for help and consolation.

It is a curious reversal of the old order. In an earlier era, the bosses like Frank Hague of New Jersey and Tom Pendergast of Missouri dispensed jobs as reward for loyal voters. The federal government has so completely taken over, in one way or another, the job operation that jobs are no worry.

Given the money availabe for staff, the geometric increase in the number of staffers is hardly a surprise. They have become a force almost as powerful as the elected Congress.