Will the presidential candidacy of Sen. Edward Kennedy divide and wreck the Democratic Party, as some predict? Or, conversely, will it invigorate and reunite the party, as other Democratic leaders prophesy?

"There will be a president named Jimmy Carter in the White House in 1981 or there will be a Republican president," flatly declares Evan Dobelle, the Carter campaign manager.

The chairman of the Democratic National Committee, John C. White, who has consistently discouraged talk of a Kennedy candidacy, says much the same thing. A Carter-Kennedy contest, he warned, "would be a mighty struggle. It would divide the party and probably make the nomination worthless."

Even Republicans profess to be worried for the Democrats. Sen. Mark Hatfield (R-Ore.) says, "I don't see how the Democratic Party could repudiate the leadership it has and then seriously go to the people and say, 'Give us another four years' with another candidate."

However, in the opinion of House Speaker Thomas P. O'Neill, the candidacy of Kennedy could "revitalize" the party. Out in California Gov. Edmund G. Brown Jr. is saying that he "looks forward" to Kennedy's entry and believes "the party will be healthier for it."

It's an argument that can be settled only by next year's general election, but if political history is any clue, there is little reason to believe that Democratic chances will be destroyed by a spirited fight for the nomination.

Over the years, there have been a number of such battles in both major parties, but after the nomination has been settled, the parties nearly always quickly close ranks behind the winner.

Only once in this century has there been an exception -- when former president Theodore Roosevelt fiercely challenged William Howard Taft, then the incumbent president, for the 1912 Republican nomination. When the Old Guard renominated Taft, the Progressives, calling foul, walked out and launched the so-called Bull Moose Party, with Roosevelt at its head. It fatally split the Republicans, leading to the election of Democrat Woodrow Wilson with less than 45 percent of the popular vote.

There has never been anything like that since. When Hatfield says he doesn't see how the Democrats could drop Carter and then ask for four more years under another candidate, he seems to have forgotten what happened in his own party in 1976.

The Republicans came within an inch of dumping their incumbent president, Gerald Ford, in favor of Ronald Reagan, and a good many members of the party still think the GOP might have won with Reagan at the head of the ticket.

Despite the supposed divisiveness of the Ford-Reagan feud, the Republicans regrouped so quickly and effectively after their convention that Ford almost won reelection. In contrast, Carter, whose nomination was assured even before the Democratic convention, had an all-time record lead of 33 points over Ford at the start of the campaign, but nearly blew it.

The GOP scenario was much the same in 1952, when Gen. Dwight Eisenhower and Sen. Robert Taft ("Mr. Republican") bitterly fought it out at Chicago for the nomination. All was peace and harmony at the Democratic convention, which virtually drafted Adlai Stevenson as its nominee. Yet, despite the Republican split, Ike won in a landslide.

In 1968, the deeply divided Democrats demonstrated that they could repudiate a sitting president (Lyndon Johnson), stage the most violent national convention in modern times, fight to the death over a Vietnam peace plank and still almost win the election. The embattled nominee, Hubert Humphrey, began his campaign 16 points behind Richard Nixon, but lost by less than 1 percent. That's how fast badly split parties can reunite once the chips are down.

If the Democrats lose next year, it's not likely to be over a Carter-Kennedy fight; for, if the polls are correct, it is no contest. Never has an incumbent president been so low in the ratings, and never has a challenger been so high.

Moreover, Kennedy, in the campaign leading up to the nomination, apparently intends to pitch his candidacy primarily on the lofty, although crucial, question of "leadership." That is a legitimate issue, yet not an inflammatory one.

It may be that Carter, as his backers swear, will never quit the race. Nevertheless, both Lyndon Johnson and Harry Truman withdrew after discouraging showings in the New Hampshire primary. It's true that Herbert Hoover, despite his unpopularity, sought reelection in 1932, but he was so crushingly defeated that other weakened presidents have chosen not to risk the same fate.