PITTSBURGH, SEPT. 7 -- Jesse L. Jackson today declared that he will declare, setting Oct. 10 as the date he will formally launch his second bid for the Democratic presidential nomination.

In Labor Day speeches here, in Cleveland and in New York, Jackson said his travels had helped him build "a new extended family" of workers, farmers, peace activists and students to join the "dispossessed" and "left out," who were the main focus of his 1984 effort.

Jackson, 45, will enter the Democratic contest as an early leader in national polls -- partly because he is the only one in the eight-person field who has sought national office before, and partly, he said, because "the more people have had a chance to see and hear me, the more my negative ratings come down."

In a nationwide poll for Time magazine's current issue, Jackson had the support of 26 percent of the Democrats interviewed, while Massachusetts Gov. Michael S. Dukakis, with 11 percent, was the only other candidate in double digits. Twenty-eight percent of the Democrats interviewed were undecided.

Jackson also has a campaign and financial structure that is far more advanced than when he began his first effort in late 1983.

That time, he spent most of the early months seeking leadership support in his base in the black community -- and not always getting it. This time, the leaders of five prominent black political organizations -- the Congressional Black Caucus, Conference of Black Mayors, National Black Conference of Local Elected Officials, National Conference of Black State Legislators, and National Congress of Black Political Women -- have endorsed him.

Spokesman Frank Watkins said 19 of the 23 black members of Congress have indicated their support. And on Tuesday, Jackson will meet with Chicago Mayor Harold Washington at the gate of a soon-to-close factory in Cicero, Ill., and "an announcement is expected." In 1984, Washington remained officially neutral during the Illinois primary, but supported Jackson at the Democratic National Convention.

Jackson's staff said they staged today's pre-announcement to capture the symbolism of Labor Day -- Jackson unveiled a worker's "bill of rights" -- and to squelch doubts in some political quarters this summer that he would be a candidate. He has spoken often of concerns about his family's security.

Jackson will make the formal announcement in Raleigh next month, site of the second convention of the National Rainbow Coalition, which he founded.

Then, and now, he will face questions about whether a black candidate, perceived by many to be outside the political mainstream, can be elected.

"The mainstream is too narrow," he said in an appearance on ABC's "Good Morning America." "We've got to turn the stream into a river and bring more people in . . . . There were 85 million people last time who didn't vote."

Asked later in the day whether political leaders are ready for a black man to be taken seriously as a presidential candidate, Jackson said: "The people are more ready than the publishers and the leaders . . . . The people were more ready in 1947 for a black baseball player than the owners . . . . "

"The party did not decide in 1932 that a man in a wheelchair could win, but the people did, and it worked. The party did not decide

in 1960 that a Catholic could

win, but the people did, and we prevailed . . . .

"If one does not vote for me because I do not make sense, then they have a right . . . . If one does not vote for me because I'm black, I leave that in God's hand . . . . "

Jackson drew loud applause from the crowd here, which was about half white and half black.

Between now and next month, Jackson will concentrate on hiring a campaign manager -- several leading prospects have turned him down -- and shoring up his financial and political base.

Campaign spokesmen said he has raised $1 million so far, and will file a report with the Federal Election Commission within the next month. In 1984, Jackson raised about $5 million, but little of it came in during the precampaign season.