Gary Hart, who rearranged the political landscape by parachuting back into the presidential race this week, is the clear leader in the fight for the 1988 Democratic nomination, while Senate Minority Leader Robert J. Dole is closing in on Vice President Bush in the GOP contest, the latest Washington Post-ABC News poll shows.

The nationwide poll of registered voters, conducted Tuesday through Thursday, also shows that a strong plurality of voters prefers to send a Democrat to the White House in 1988, but given a choice between former senator Hart (Colo.) and Bush, the current front-runners, they pick the Republican.

The poll shows:Hart was the favorite of 30 percent of the registered Democrats sampled, followed by Jesse L. Jackson at 20 percent, Massachusetts Gov. Michael S. Dukakis at 15 percent, Sen. Paul Simon (Ill.) at 8 percent, Sen. Albert Gore Jr. (Tenn.) at 5 percent and Rep. Richard A. Gephardt (Mo.) and former Arizona governor Bruce Babbitt at 2 percent apiece. Eighteen percent were undecided.

In a nationwide Washington Post-ABC News poll taken before he got out of the race last spring, Hart was supported by 47 percent of Democrats interviewed.

Bush leads Dole by 44 to 35 percent among registered Republicans sampled. But his margin of 9 percentage points is about half of what it has been in most national polls taken this fall. The latest findings indicate that Bush got no immediate benefit from his high-visibility meeting with Soviet leader Mikhail Gorbachev during last week's summit and his strong support for the arms-control treaty that was signed at the summit.

The other Republican contenders were all well back in single digits: Rep. Jack Kemp (N.Y.) at 6 percent, Pat Robertson at 4 percent (down from 9 percent in October), former secretary of state Alexander M. Haig Jr. at 4 percent and former Delaware governor Pierre S. (Pete) du Pont IV at 3 percent.

The Republicans received no clear boost from the summit between Gorbachev and President Reagan. Voters said they trusted the Democrats over the Republicans, by 49 to 39 percent, to handle the main problems the nation will face over the next few years. That 10-point gap matches the largest the Democrats have enjoyed on that question at any time in the last five years. It suggests that the economic anxieties either triggered or confirmed by the Oct. 19 stock market collapse have had more of an influence on voter attitudes than the glow of the summit. Just prior to the plunge, Democrats had only a one-point edge on that question.

By 48 to 33 percent, voters said that if the election were held today, they would prefer a Democrat over a Republican, a preference suggesting a strong desire for political change. But when the question was asked about specific candidates, the results flip. Republican front-runner Bush is preferred over Democratic front-runner Hart, 52 to 39 percent, in a head-to-head matchup. Last January, when voters were given that same choice, they picked Hart over Bush by 58 to 35 percent.

Hart's reentry into a race he quit last May, following reports of his relationship with Miami model Donna Rice, had the approval of 52 percent of the voters sampled and the disapproval of 44 percent. Younger voters were far more likely to welcome Hart back than older voters (66 percent of 18 to 30 year olds supported his return, compared with 47 percent of those over 45 years old), and men were slightly more supportive than women.

The poll suggest a quandary for the Democrats: Their leader in the polls is someone who may be neither electable nor nominatable but who has a strong enough base of support to be a major factor in the primaries.

"Here you have the closest thing we have seen in a long time in our party to a political pariah -- and he's leading in the polls," said Richard Moe, a longtime party activist and an adviser to Gephardt. "It's a bizarre situation. My guess is that he's at his peak right now. He's a media phenomenon. I don't think he has a case he can sell in the long run. Lord knows that people have been successful running against Washington, but with Hart there isn't a political figure in the party who will go near him. You cannot hope to win, much less govern, without the support of the political community."

Other Democratic insiders, while sharing Moe's scorn for Hart, are less certain that he will fade quickly. "He's clearly outsmarted a whole lot of us," said Robert Beckel, 1984 campaign manager for former vice president Walter F. Mondale, marveling at Hart's initial success with his antiestablishment, I-am-a-victim, don't-let-them-take-your-vote-away-from-you message.

Beckel said the other candidates will have to walk a careful line in attacking Hart. If they raise the character or morality issue, he said, they may provoke sympathy for his argument that the press invaded his privacy. If they call him arrogant, or selfish, or a spoiler, they may heighten the appeal of his antiestablishment message.

"The reason he's not electable is the thing we discovered back in the red phone days," Beckel said, referring to a 1984 Mondale ad that asked which candidate would do better handling a nuclear crisis. "The issues are unpredictability, unsteadiness, recklessness. You put the word 'uncertainty' in the same sentence with the word 'president' and it doesn't fit."

Other polls taken this week show Hart trailing Dukakis in New Hampshire and saddled with high disapproval ratings there and in Iowa. If Hart does not do well in the opening states, speculated Simon pollster Paul Maslin, his candidacy could fade quickly.

On the Republican side, several analysts attributed Dole's gain at the expense of Bush to a slow but steady increase in his name recognition. The Republican sample in the Post-ABC News poll was of 303 voters, meaning that the margin of error was 6 percent. The Democratic sample size was 318 voters with an identical margin of error.Polling director Richard Morin and analyst Kenneth E. John contributed to this report.