Both the current dean of the law school and Mr. Biden's professor today played down the incident of plagiarism. [ Page A23. ] Brushing aside any suggestion that he might be forced to withdraw from the Presidential race, Mr. Biden declared at the news conference, ''I'm in the race to stay, I'm in the race to win, and here I come.'' Blames Rivals

Mr. Biden also suggested that the recent damaging information about him had originated with other campaigns, which he did not identify, and that it had emerged now because he was enjoying a chance in the limelight with the Bork hearings.

''Look, I'm a big boy,'' he said. ''I've been in politics for 15 years. This is not my style. If they want to do it this way, so be it.''

The file distributed by the Senator included a law school faculty report, dated Dec. 1, 1965, that concluded that Mr. Biden had ''used five pages from a published law review article without quotation or attribution'' and that he ought to be failed in the legal methods course for which he had submitted the 15-page paper.

The plagiarized article, ''Tortious Acts as a Basis for Jurisdiction in Products Liability Cases,'' was published in the Fordham Law Review of May 1965. Mr. Biden drew large chunks of heavy legal prose directly from it, including such sentences as: ''The trend of judicial opinion in various jurisdictions has been that the breach of an implied warranty of fitness is actionable without privity, because it is a tortious wrong upon which suit may be brought by a non-contracting party.'' Just One Footnote