In a chat at his White House office the other day, Vice President Walter Mondale got to musing on the changes that had taken place in the Congress since he first came to Washington a dozen years ago. "The Congress," he said, "is a different place now.

"When I first came the place was dominated by a few whales - people like Sen. Richard Russell of Georgia. The South was solid, and Southerners held most of the powerful positions. Some subjects were taboo. For instance, you just didn't debate on defense.

"Now," he continued, "the whales are gone. The South is strong but the Congress is much more national. The leading figures come from everywhere - Muskie of Maine, Al Ullman of Oregon, Gaylord Nelson of Wisconsin, Ribicoff of Connecticut, Speaker Tip O'Neill of Massachusetts. All subjects are open for debate, especially defense. You have a lot of people around who like to kick the system."

In describing that change, Mondale went to the heart of the trouble the Carter administration has recently been having with the Democratic Congress. The difficulty is not, as the President and his men are often pleased to assert, with the special interests.

The difficulty is with the new spirit pervading the Congress. It is a maverick spirit that features independence over cooperation, getting on over going along, kicking the system over supporting it. It is the same spirit that dominates the Carter administration and that brought Carter to the presidency in the first place.

When he is working with that prevailing spirit, Carter tends to enjoy success - even striking success - in the Congress. Thus his basic reorganization plan for the government shot through both houses. Similarly he will be getting approval for a new Department of Energy in a period far shorter than that required to set up any previous new department.

The victories he has scored in the past 10 days of intense battling fit the same pattern. He was able to mobilize a fairly large majority on the House Ways and Means Committee to sustain a key proposal of his energy program: for wellhead taxes to be paid by the oil companies out of the higher profits they will realize from rising prices. He won that fight by organizing diffuse independent sentiment against the big, bad oil companies.

Another Carter victory last week came in the bitterly fought contest over water projects. The leading pork-barrel specialists in the Senate (Robert Byrd of West Virginia and Russell Long of Louisiana) and the House Majority Leader had all rebelled against a Carter move to kill 18 water projects. The Senate voted overwhelmingly to authorize the projects.

In the House, Carter lined up the freshman and sophomore members in a revolt against the old-time pork-barrel system. The system prevailed in a 214 to 194 vote, with most Democrats going against the President in voting to fund the water projects. But of freshman and sophomore Democrats, 68 supported Carter while only 38 broke with the administration.

Their support, and the close vote that resulted, caused the Senate to pull back. Now the prospect is for agreement on a limited number of water projects and a semi-victory, at least, for the administration.

But Carter failed entirely when he did not run with the anti-system sentiment. His proposal for a tax on gasoline, for instance, looked like a straightout assault on the millions of ordinary Americans who use their cars in day-to-day living. His proposal for rebates on gas-saving cars looked like a bonus for the foreign producers. In both cases the proposals were very short on details. So both proposals went down in the Ways and Means Committee by lopsided votes.

What all this says to me is that there are the makings of a Carter base in the Congress. He can build a hard core of support, particularly among the newer members who march to the same anti-system music that marks his cadence.

But in order to attract this hard core of support, the President will have to do some things he does not seem to like much. He will have to work very hard at the backscratching and patronage and flattery that are the cement of all legislative majorities. He will have to stay on the reform side of issues. He will also have to fill out details of programs instead of pushing them through quickly without full consideration of the fine points. Lastly, he will have to forgo the big-think foreign policy issues that so entrance him and concentrate on the domestic issues that are so much more important.