As Ronald Reagan's fortunes have fallen in the northern industrial states, his strategists have discovered newfound optimism in what they see as sizeable Reagan gains in President Carter's southern base.

The much-hearlded key states of the North and the Great Lakes may not turn out to be so key after all, Reagan officials now say. "The fulcrum of this campaign has shifted to the South," says one senior Reagan strategist. "That's where the election will be won or lost."

Reagan now seems in a solid position to carry Florida and probably Texas as well, according to sources in both camps. These two states, the largest in the South, both went for Carter in 1976, and he could not have been elected without them. Reagan is also running close to Carter in as many as seven southern and border states, according to the Reagan camp's calculations. sCarter officials contest some of those figures.

In 1976, when Carter narrowly defeated President Ford, he carried all of the southern and border states except Virginia, which was Ford's in 1976 and which is giving Reagan a comfortable lead in 1980.

Reagan officials maintain that the Republican nominee's southern prospects firmed up even as his northern support was softening because of the differences in the way the war-and-peace issue has played in the two regions. They said the controversy caused Reagan considerable problems in the industrial North but minimal problems in the South -- and in some conservative areas, it may even have backfired on Carter by reinforcing Reagan's image as the more hawkish of the two candidates.

Carter officials remain confident the South will stay overwhelmingly with the president and they quickly deny that the war-and-peace issue has back-fired on the president in any area in the South.

"Carter is firmer in his base today than Ronald Reagan is in Oregon and Washington out West," says Jim Free, Carter's deputy chairman for the southern region. "I'm not bulling you. Reagan has not appreciably moved in the South. But if they want to concentrate on the South, then bravo to them."

Carter strategists do not believe the southern poll figures put forward by their Reagan counterparts. They think Reagan's advisers are seeking mainly to tie Carter down in his own base.

The Reagan officials offer a counterargument. "I never would have thought it possible," said one senior Reagan strategist, "but it is now possible that we could lose Pennsylvania, Ohio, Michiagn and Illinois and still win this election -- if we pick up some of these states in the South that are now looking good."

Reagan losses in Michigan and Illinois, the two key northern states that Ford carried in 1976, could be offset by Reagan victories in Florida and Texas. Reagan's margin of victory could then come through triumphs in several other southern and border states, his advisers believe.

Here's how both camps see the southern and border regions at this time:

FLORIDA -- Reagan's poll, by his public opinion expert, Richard Wirthlin, shows him boosting his lead by 4 percentage points between mid-September and mid-October. Reagan now leads Carter by a whopping 12 points, 43 percent to 31 percent, according to his survey. Carter pollster Patrick Caddell maintains that Reagan's lead in the state is about 6 points. But some Carter campaign officials are known to concede that Reagan seems to hve the state won. One top state Democrat said sadly: "Florida is Reagan's. I hate to say it but I think it's true."

TEXAS -- Reagan's poll shows him with a 4- or 5-point lead and his senior advisers say they expect a heavy Republican turnout in the Houston suburbs to keep it that way. Carter officials maintain that Texas is still "very tight," although they concede the edge to Reagan.

ALABAMA -- Reagan's poll had shown him ahead in August; but after his Ku Klux Klan gaffe, he plunged to a 15-point deficit within 10 days. Now, according to a senior Reagan official, Reagan has climbed back within 5 points of Carter, with 47 percent for the president, 42 for Reagan, 4 for independent candidate John B. Anderson and 7 percent undecided.

Louisiana -- Reagan's recent survey shows him with a 2-point lead; Carter's strategists say their polls show the state is "very close and Reagan is not leading."

MISSISSIPPI -- Reagan's poll shows this is one southern state where he has slipped, from a 3-point lead in mid-September to a 6-point deficit now. Strangely, it is the Carter strategists who contend the president the president's lead is nowhere near that large and that the state is about even.

SOUTH CAROLINA -- Both sides consider the state very close.

TENNESSEE -- Both sides consider this state very close. In 1976, this was one of Carter's strongest states.

KENTUCKY -- Reagan's poll shows him having climbed to within 1 point of Carter; Cater's poll shows it close but with Reagan several points behind.

MISSOURI -- Reagan is now just 4 points behind and has climbed from 33 percent to 41 percent in the last four weeks while Anderson's vote has fallen from 13 percent to 6 percent and the undecided vote is down to just 7 percent. Carter officials say that is about right.

Both sides agree that North Carolina and Georgia are Carter's and probably Maryland too. Reagan holds a sizeable lead in Virginia.