Ronald Reagan's campaign has become locked in middle gear in the crucial northern industrial states, and his advisers are shifting to more aggressive tactics in an effort to regain the momentum they once had.

The prospect of victory on Election Day is now clearly at stake, they concede.

Six weeks ago, Reagan's pollster and political adviser, Richard Wirthlin, said that unless Reagan went into the final stages of the campaign with "a cushion of 6 to 8 points, you'd have to worry about the incumbent pulling it out."

But according to a number of polls, including those taken by The Washington Post between Sept. 26 and Oct. 5, Carter and Reagan are running even in seven key states. A margin of 3 percentage points or less separates the Democratic and Republican presidential nominees.

The failure of the Reagan campaign to convert his early lead over Carter into solid positions of strength in such key states as Pennsylvania, Ohio, New Jersey, Illinois and Michigan is viewed by Democrats -- and some Republicans -- as evidence that Reagan has failed to look up the election, and that victory may now elude him.

One major reason for Reagan's problems in these states has been President Carter's success -- through television advertising and his own rhetoric -- in raising doubts about Reagan's ability to keep the nation out of war.

"Carter has shifted the issue from the economy to war and peace," said one Michigan Republican, "and Reagan has to find the way to shift it back."

There is no panic in the Reagan high command. Wirthlin said yesterday that after a slump last week in many of the key industrial state battlegrounds, "Reagan has come back up a point or two in the latest stuff I've seen." Stuart Spencer, the top man in Reagan's traveling entourage, is offering to bet substantial sums that Reagan will carry Texas and at least two other southern states from a list of five that went for Carter in 1976 and are close to even now.

Such inroads in Carter's southern base would compensate for the loss of any of the industrial states, as Reagan's own stronghold in the West appears to be holding firm.

Still, the Reagan camp made several decisions and announcements yesterday that represent a recognition of his problems.

Aides said that Reagan will make a televised half-hour talk to the country -- perhaps as early as next Monday -- on the issue of war and peace. In it, Reagan will rebut Carter's claims that he would have sent American forces to several international hot spots in recent years. He also will defend his call for a bigger defense budget and the scrapping of SALT II in favor of fresh arms-controls talks with the Soviets.

Polls have shown that Reagan is losing the election to Carter among women voters, presumably on grounds that they are less confident of his ability to keep the United States out of War. tA secondary factor may be his opposition to the Equal Rights Amendment -- a stand that drew boos at a rally in Michigan yesterday and in several other recent Reagan appearances.

Polls also show Reagan making very slight inroads on the black vote so far -- a failure that hurts his chances of carrying states from Michigan to Mississippi.

Finally, the Reagan high command is reconsidering the decision to shun any debate with Carter that excludes Anderson. Reagan does not want to be the one who shuts the door on Anderson, whose support damages Carter much more than it does Reagan. But Spencer and other key advisers are swinging over to the view that a face-to-face debate with Carter may offer Reagan his best chance to be "both reassuring and presidential," as one man put it, and thus do what he has so far failed to do -- lock up the election.

As the race for the presidency has tightened, divisions within the Reagan camp have widened over just what the candidate ought to do to regain his momentum.A number of senior advisers, both in Reagan's headquarters and in a number of state field offices, are concerned that the campaign has been too complacent and cautious.

"There's no question in my mind that the campaign has not been aggressive enough," said one Reagan adviser. "We've been just sitting back and taking too many lumps."

A number of Reagan's advisers believe that he should debate Carter face to face. "With the numbers as close as they are now, I don't think there's any question but that a debate would help Reagan," said one adviser.

Others in the Reagan high command are unhappy that the candidate has decided against participating in citizen question-and-answer programs that would be, in effect, a call-in video version of the Carter town meetings.

George Bush, Reagan's running mate, used these "Ask George Bush" television specials with considerable success during the late primary season, when he defeated Reagan in Pennsylvania and Michigan. The shows are continuing -- they are called "Ask Reagan-Bush," but they apparently will star only Bush, never Reagan, according to Reagan advisers.

Reagan campaigned in Michigan yesterday with some of the glitterati of the Detroit convention that skyrocketed him into his late summer lead. At his side were his onetime adversary and current running mate, Bush; his onetime adversary and almost running mate, former President Ford; and Michigan's popular, moderate Republican governor, William G. Milliken. They were convened at a rally in the Detroit suburb of Birmingham. The massing of the GOP's biggest names was an effort to nail down wavering independent and ticket-splitter support in one of the states holding the key to the election.

Reagan strategists are more hopeful about the trends in Michigan than they are about their prospects in a number of other key states. Michigan is one of the few big states where the Carter and Reagan officials differ significantly in their assessments. Carter officials say their polls show the president leading in Michigan, which went for homestate boy Ford in 1976.

Reagan's prospects for victory seem linked to the question of whether he will be able to improve his standing among the women voters, especially on the war-peace issue.

But there is also evidence that Carter is not becoming the immediate beneficiary of these concerns about war and peace. Some of the voters have shifted away from Reagan -- but to undecided status, rather than to Carter, perhaps as a result of concerns over the occasionally emotional and strident way in which Carter attacked Reagan on the issue as he initially sought to dramatize the difference between the two candidates.

In Ohio, the latest poll taken by the Reagan campaign, according to one Reagan official, showed Reagan at 34 percent, Carter at 31 percent, Anderson at 12 percent, and 23 percent undecided. The Reagan official said this meant that Reagan had lost several points from their previous poll, but that it was the undecided category, not Carter, that gained.

Reagan campaign officials believe the war and peace issue is causing less damage to the Republican nominee in two of the key states in the south, Florida and Texas, than it is in the northern industrial states.

"In Florida," explained Herb Harmon, Reagan's state coordinator, "we've got a lot of hawkish, patriotic people who may be out in their barns sharpening their swords right now."

One prominent Democratic pollster, Peter Hart, views the war and peace issue as a major, and perhaps cruicial, factor in this year's campaign. But he says that it has been around as an underlying concern of voters throughout the fall campaign, and now, as the campaign nears an end, it is looming crucial.

"I really saw Ronald Regan very much in the driver's seat even into mid-September," Hart said. But he says he has come to feel that Reagan will not win the election because he has been unable to increase his standing in the public opinion surveys despite considerable doubts about Carter.

"Think of it as chips on a poker table," says Hart. "Reagan should have 35 chips in his hand right now and another 15 on the table. But instead, he has 15 in hand and 35 on the table."

As the Reagan strategists try to kick their campaign back into high gear, their counterparts in the Carter campaign are beginning to talk confidently about their own prospects. They are so confident, in fact, that they have taken to making jokes about the Reagan camp's fondness for expressing publicly their fears of a surprise international triumph by Carter in October.

"You know that 'October surprise' the Republicans have been predicting Jimmy Carter would spring on them?" an aide to Vice President Mondale asked the other day. "Well, now they know what it is: Carter's going to win the election."