The simple political reality behind the eroding support for Supreme Court nominee Robert H. Bork is that the bearded, former Yale professor did not win the hearts of southerners.

The South and its "Boll Weevils" gave President Reagan his margin of victory on one major legislative initiative after another in his first term. And it was to southern Democrats that the president looked again for crucial votes for Bork's confirmation by a Democratic-controlled Senate.

But back home in the South, voters initially favorable to Bork as a conservative backed by Reagan turned increasingly lukewarm as his confirmation hearings progressed, according to regional polls and interviews with a range of southern voters and political analysts.

Bork's assertion in his confirmation hearings that the Constitution does not protect a general right to privacy struck at an ingrained southern desire to be left alone, the analysts said. And his record of early opposition to the Civil Rights Act inflamed black voters, the most potent element of the southern Democratic coalition.

What's more, southern cultural factors apparently came into play against Bork. Television stations and newspapers throughout the region gave heavy coverage to Bork's youthful immersion in socialism and his travels across the ideological spectrum. A Birmingham, Ala., newspaper columnist, expressing support for Bork, nonetheless emphasized his "bizarre" appearance, an apparent reference to his beard.

"From a Texas perspective," said Jack Martin, 1988 campaign director for Sen. Lloyd Bentsen (D), who announced his opposition to Bork on Friday, "here's a former Yale professor who wants to deny these people their right to privacy. He might as well be saying he wants to deny their right to bear arms."

A critical factor was the impassioned fear expressed by black voters who, as one-quarter of the electorate in the Deep South, provided the margin of victory for four southern Democratic freshmen elected to the Senate last November.

"For virtually all of the southern Democrats, their reelection now depends on mobilizing large numbers of blacks and a minority of whites," said Merle Black, a University of North Carolina political scientist and coauthor with his brother, Earl, of "Politics and Society in the South," a respected study of the region. "For a southern Democrat to vote to confirm Bork would be to break up his previous election coalition. And it wouldn't gain much from the conservatives because they're going to vote Republican anyway."

Although most southern Democrats reported a preponderance of letters and phone calls favoring Bork, political analysts said that no more than 20 percent of the electorate was activated by the nomination, either pro or con. Those mobilized in Bork's favor, such as the religious right, likely have joined the Republican camp and generally will not support a Democrat under any circumstance.

Sen. Howell Heflin (D-Ala.), one of three undecided members of the Judiciary Committee, which is to vote Tuesday on the nomination, told of being cornered last weekend in the Montgomery, Ala., airport by a man who said he was for Bork.

Heflin asked, "Are you a Republican or a Democrat?"

"Well, I'm speaking to you as a Democrat," was the answer.

"The vocal people are 6 or 7 to 1 for him," Heflin said last week. "But the polls and my reading of it tell me it's all even-steven."

A poll published last week in the Atlanta Constitution showed voters in 12 southern states opposed to Bork's confirmation, 51 percent to 31 percent, with the rest expressing no opinion. The opposition was firm among whites as well as blacks, and even among those who described themselves as conservatives.

Moreover, a disproportionate share of the pro-Bork mail to many southern offices came from out of state and appeared to be form letters, diminishing its political punch. Jerry Ray, press secretary to Heflin, said he pulled a handful of 61 letters from the bulging mailbag one day last week, and counted 57 from outside Alabama; only four from the state. Those four were evenly divided on Bork, he said.

With the bulk of the white electorate lukewarm at best, lawmakers are free to respond to the extraordinarily intense opposition of southern blacks to Bork, who in 1964 had criticized the Civil Rights Act as unconstitutional and who continues to question the constitutionality of the Supreme Court's one-person, one-vote reapportionment ruling that is a bedrock of black political ascendancy in the South.

Although Bork later recanted his opposition to the Civil Rights Act and vowed in his confirmation hearings not to disturb the one-man, one-vote precedent, the fears of southern blacks were unabated.

Illustrative of the black opposition was an effort mounted by the Alabama Democratic Conference, a formidable, statewide coalition of black religious and civic groups. "We had meetings in every district of the state," said the group's field director, Jerome Gray. "We took kits on how to write letters. . . . We provided envelopes and stamps. We gave every county a goal for how many letters they should generate. Most of them went over it."

Asked why Bork's assertions in favor of civil rights during his confirmation hearings had little effect, Gray responded, "We know a tree by the fruit it bears, and the fruit he's been bearing in the past has not been good for us."

The help given to southern Democrats by black voters is undeniable. According to exit polls, blacks supplied the margin of victory in November for Sens. Richard C. Shelby (D-Ala.), Wyche Fowler Jr. (D-Ga.), John Breaux (D-La.) and Terry Sanford (D-N.C.). Each got only a minority of the white vote, and close to 90 percent of the black vote.

The trend reflects the changing political character of the region since passage of the 1965 Voting Rights Act. The percentage of black voters within the Deep South's electorate has grown from 11 percent to 24 percent since 1960.

"You don't change the demography of the voting population of those states and expect them to ignore it," a member of the White House strategy team said last week as southern Democrats began shifting into the "no" column.

The lack of intense feeling among southern conservatives is perhaps the biggest surprise of the Bork debate. A number of southern politicians and observers said constituents appeared at first favorable to Bork, as Reagan's nominee. But the more they saw, the less they liked.

"This man was not the southerner's idea of a hero," said Atlanta pollster Claibourne Darden. "We just got through making Ollie North our hero and here comes a real dry, peculiar-looking fellow who won the hearts of very few."

In this light, Darden and others cautioned against interpreting the vote simply as a sign of expanded black political power in the South.

"This is still a region where politicians are careful what they give the black vote, so they don't alienate the white vote," he said. "So the folks who vote against Bork will say to their black voters, 'Now we did something for you,' and they won't get too much negative recourse from the whites. That's what you call a safe issue."