Democratic presidential aspirants Walter F. Mondale and Sen. Gary Hart (D-Colo.) arrived early for the Massachusetts Democratic Convention to launch new, sharp attacks on President Reagan's policies, while Sen. Alan Cranston (D-Calif.) came early to hype the importance of a preferential straw poll that will be taken among delegates here.

Former vice president Mondale charged that the Reagan administration had treated "cynically" and in an "uncaring, unresponsive" way requests for help from residents of Woburn, which has been beset by toxic waste problems.

He said he had stopped by Woburn on his way here and saw that it has "become a time bomb." In the part of the town that is polluted by toxic waste, he said, the cancer rate is 15 times that of the national average.

Mondale said he talked to three mothers, each of whom had recently lost children to leukemia.

"They have asked for help from their government and they got little or none of it," he said. " . . . This administration is dealing cynically with those basic demands." The Reagan administration, Mondale added, had been "taken over by administrators . . . who were committed to fighting for the side of the poisoners."

Hart announced plans to circulate a letter among delegates urging Reagan not to "extend the nuclear arms race into space."

Reagan recently spoke of installing defensive weapons in space that could destroy enemy missiles launched against the United States.

" . . . It would surely move us a giant step closer to nuclear war," Hart's letter to the president warned. "We call on you, instead, to negotiate a verifiable ban on weapons in space."

Cranston, who has invested heavily in campaign time and money in the hope of doing well in the delegate straw poll here, told a news conference that he considered the poll "quite significant . . . a major test." He said he thinks he will gain "credibility out of a good showing here. There have been some doubts about my ability to be competitive."

Hart and Mondale, on the other hand, played down the straw poll's significance.

"I think it will be about as significant as the others that have been conducted," Hart said. "For the time being, they give people something to talk about, write about," Hart said, adding that the first presidential primary is a year away.

The straw poll here has been blurred by the effort of the AFL-CIO to have its members vote for "Jobs" instead of for one of the candidates by name.

State Democratic Chairman Chester Atkins, asked what the significance would be if "Jobs" finished first, replied, "If a plurality votes for 'Jobs' it would indicate a strong commitment to the policies centered around economic renewal and that none of the candidates have fired the imagination of those delegates.