Republican confidence that President Reagan, who campaigns this week in the South, can do very well in this crucial region in November is based on the following assumptions, say Southern campaign strategists and state party officials:
* That Democratic attacks on the fairness and sincerity of President Reagan are likely to backfire in the battle for Southern votes.
* That Democratic vice-presidential candidate Geraldine A. Ferraro is too liberal to suit most Southerners, regardless of her appeal as a woman.
* That Southern support of a strong military will override concerns Democrats will try to raise over record federal deficits.
But the Republicans expect the Mondale-Ferraro team to mount a major effort in the South, since there is little likelihood the Democrats can win in November without carrying at least some states in the region.
Confident as they are, there is an edge of concern among Republicans.
''We could lose the South,'' says Georgia's Republican state chairman Bob Bell. ''It's ours to lose if we go to sleep.''
President Reagan's campaign appearances this week in Texas and Thursday in Georgia are aimed at helping make sure his supporters in a region he carried in l980 do not go to sleep.
''Anybody needs to be reminded that you know they're there and you respect them,'' says Lou Kitchin, Southern regional campaign director for the GOP. By coming South now, he adds, the President is saying: ''I appreciate your support and need it again.''
US Rep. Carroll A. Campbell Jr. (R) of South Carolina says the President ''has to continue to pay attention to the South.''
Mr. Reagan must ''go on the offensive - very hard'' against Democratic claims , says Congressman Campbell, who heads the Reagan-Bush campaign in his state. He says the President should press the Democrats on their plans to cut military spending without cutting military strength, and on balancing the budget without cutting domestic spending, he says.
While various Southern Republicans welcome a debate on the programs and philosophies of the two parties, they say attacks on Reagan as a person could hurt the Democrats.
''It is not legitimate at all to question the fairness and sincerity of Ronald Reagan,'' says Bill Harris of Alabama, chairman of the Southern Republican State Chairman's Association. Referring to attacks on the President by Democratics in San Francisco last week, he said: ''I think it was a mean-spirited convention.'' Southern Campaign director Kitchin says the President ''exudes sincerity. The people in the South really look for that in a man.''
The Ferraro factor may be good politics in some parts of the nation, but is unlikely to influence many votes in the South, according to the Republicans. They are counting on their ability to make voters aware of what they call a voting record too liberal for many Southerners.
They are already beginning to use terms like ''liberal,'' ''super-liberal'' and ''ultra-liberal'' to describe her. And they are hoping Vice President Bush's record of foreign policy experience outshines hers. In many areas of the South registered Democrats far outnumber registered Republicans. And intense voter registration of likely Democratic voters, especially blacks, is under way.
What the party needs to do in the region, say Southern GOP officials, is mount a grass-roots challenge to register likely Republican voters; identify registered Reagan voters; and to get both groups to the polls on election day.
Such efforts have begun in several Southern states. In Florida a major voter registration drive is underway in a number of areas including predominantly Republican sections of Miami, says Tommy Thomas, Reagan-Bush campaign chairman in the state.
Registration drives also are under way in Georgia, Texas, and North Carolina.
But with Reagan apparently ahead in Florida, why is Mr. Thomas working so hard? He says he wants to increase the chances of two and possibly three Republican congressional candidates to catch the advantage of the Reagan coattails and defeat Democratic incumbents.
''Hard work, all hard work,'' is what is needed now, says Martha Weisend of the Reagan-Bush campaign in Texas. On fund raising, she said, ''we have not gone 'gung ho' as of yet.'' Reagan carried Texas by 55 to 41 percent against President Carter four years ago. He carried Florida 56 to 39. Some other Southern states he won only narrowly, however.
In the 17-state region of the Southern Governor's Association, President Reagan lost only Georgia, West Virginia, Maryland, and the District of Columbia.