A week away from the start of the most critical session of Congress since his first as president, Ronald Reagan has just stopped short of the crucial decision that could spell success or failure for himself and his party this year. At both ends of Pennsylvania Avenue, officials are waiting to see whether, when the crunch comes, he will play his own game of legislative strategy or join in forging a comprehensive Republican game plan.

The crux of the strategy question -- as of almost everything in Washington this year -- is the budget-and-tax issue: whether Reagan will "play chicken" with Congress again this year by holding out to the last against revenue increases and defense cuts, or work with his party leaders in the Senate and House to avoid a budget crisis a month before the midterm election.

The answer, in both governmental and political terms, would seem obvious. Key GOP legislators were encouraged by last week's overtures from chief of staff Donald T. Regan for a unified approach. But the bottom- line decisions are still to come.

The choice was put in stark terms Jan. 5, when former Reagan White House political director Edward J. Rollins appeared on CBS's "Face the Nation." "I think," he said, "it's very, very important for the White House to sit down with the (Republican) leadership of the House and the Senate, decide what the priorities are in 1986, what is going to be the national agenda, what are they going to run on in 1986.

"If they do not," Rollins warned, "if they end up in a year of chaos, as they did with battles with the Senate Republicans in August and the House Republicans in December, then I think you're going to have some serious problems."

Rollins was responding to a question about the GOP's prospects in the midterm election, with 22 Republican Senate seats at stake and control of the Senate in jeopardy. But, as he noted, the president has as much on the line as any Republican senator or representative who must face the voters.

"It's very, very important that he personally step right into this," Rollins said, "because . . . whether he's on the ballot or not . . . this is going to be a referendum on him, and it's certainly going to be very, very indicative of whether he can . . . lead effectively through the remainder of his term."

Rollins' view is echoed by some of his former colleagues still on the White House payroll. Their view is that Reagan has little time left for posturing if he is going to shape the product of this Congress and avoid coming to blows with his fellow Republicans on Capitol Hill

Last year, when Reagan played Rambo, he was blamed by Senate Republican leaders in August for undercutting their budget package and by House Republican leaders in December for sabotaging their efforts on the tax revision bill. That kind of infighting could poison Republican prospects in the midterm campaign; if it is to be avoided, Reagan will have to decide to deal early this year. The budget timetable is inexorable.

In three weeks, Reagan will submit his own budget for fiscal 1987, holding the line on taxes, providing for 3 percent growth in defense spending above inflation, and outlining proposed cuts of $50 billion or more in unprotected domestic programs, in order to reach the Gramm-Rudman deficit limit of $144 billion.

That budget will produce screams of pain and outrage from many members of Congress and domestic interest groups. But Reagan will have positioned himself to say, "You guys (in Congress) set that limit; I have showed I can live with it; now it's up to you."

The president's position will be further reinforced by March 1, when the first automatic "sequestering" of funds under Gramm-Rudman takes place. That involves cutting $11.7 billion from current fiscal 1986 spending. White House aides expect Reagan to allow half that total to come from defense accounts, as the law provides, thus demonstrating fairly cheaply that he is willing to take the bitter with the sweet.

But then comes crunch time. Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) wants to meet the next Gramm-Rudman deadline by getting the fiscal 1987 budget resolution onto the Senate floor in April. Domenici has told the White House that if Reagan wants to be part of the process, he has to be ready to talk turkey on what he will accept in the way of defense cuts and how much and what kind of fresh revenues he will allow to be thrown into the mix.

In his news conference comments Jan. 7, Reagan was saying "not now" to both propositions. By April, however, with both the 1987 budget and his prized tax revision bill headed for the Senate floor, there will be great pressure on him to think again.

Personally, I doubt the answer will change. But if it doesn't, those Republican hopes of midterm election gains may go out the window. Republicans win only when they are delivering a clear and unified national message. To believe they can squander 1986 on internal fights over budgets and taxes and still prevail in November is really the impossible dream.