President Carter seems likely to wind up his dealings with Congress later this year in an eleventh-hour gambling session that has seen more-experienced presidents go under - a legislative poker game in which Sen. Russell B. Long (D-La.) holds all the aces.

Although it't too early to tell precisely how things will turn out, most of the signs so far indicate the wily Senate Finance Committee chairman is apt to head virtually all the House-Senate conference committees that will handle Carter's major legislative proposals.

And with Congress adjourning early because of the November elections, it's likely that most of these bills will be in conference at the same time. As a result, Long will have even more opportunity than he's had before for under-the-table bargaining on key legislative issues.

"At the end, it's just going to be Russell Long, sitting there having the major say on most of Jimmy Carter's programs," says one longtime congressional observer who has tracked the Louisianan's dealings with previous presidents.

"If Carter thinks Congress has been difficult to deal with so far, he hasn't seen anything yet. Russell Long will be sitting there, saying what goes and doesn't go in all his key legislation. The president won't even know what hit him until it's all over," he said.

The ploy isn't a new one. Almost every year for the past decade or so, Long has been sitting in the catbird seat, trading in this or that chip for heat-of-the-finish compromises, and effectively reshaping entire presidential programs in a few afternoons.

Traditionally, Long's stakes have included a big presidential tax-cut bill, some sort of Social Security legislation, and possibly a measure involving revenue-sharing - enough to enable him to raise the ante easily to include this or that piece of special-interest legislation.

But this year, lingering statements in the counterpart Ways and Means panel and the House-Senate ennergy conference committees have enlarged the end-of-session pot to include virtually all of Carter's big legislative proposals. Among those on the list:

The big $20 billion tax-reduction bill. Because of a fight over the "reform" provisionh - and a Republican bid to cut capital gains taxes - it'll be lucky to make it over to the Senate before autumn.

The energy bill. Although the conferees are scheduled to meet on Thursday, there's no indication yet that they'll finish quickly. Final decisions on the energy package may come in September or October.

Welfare reform. Ways and Means is only beginning work on Carter's big welfare package, and probably won't take it up in earnest until after members finish the president's tax package.

Tuition tax credit legislation. The House in this case has passed a tuition credit measure, but the bill still is waiting for Senate action - in line with a slew of other bills.

Hospital cost-containment. This measure, crucial to Carter's pledge for a national health insurance program, isn't exactly zipping through the House. Long favors a different approach.

Miscellaneous measures. Add to this a series of other bills important to the administration: the tax portion of the aircraft noise-abatement measure, the tax treatment of Americans living abroad, a bill affecting income taxes paid by heirs, and oil-import fees or quotas.

Moreover, on many of these issues, Long has given only a hint of where his views lie - leaving his options open for last-minute trading later in the session. (On the tax bill, for example, he's said only he think $25 billion is a good net figure. No word about details).

Depending upon how it's calculated, the listing would leave Long between $30 billion and $60 billion in stakes with which to play his end-of-session game. It's enough to make an inexperienced player lose his shirt - or make a clever one a king.