Sen. Muriel Humphrey led Rep. Don Fraser 41-to-40 percent when Minnesota Democrats were asked by the Minneapolis Tribune who would be the strongest candidate for their party in November's Senate election.

The statewide poll, published yesterday runs contrary to an earlier sampling by the Fraser campaign, which found him the frontrunner by nearly 2 to 1 for Democratic Party convention endorsement in June. But a Fraser spokesperson said the congressman regarded the Tribune poll as "positive." Fraser was saiding to be gaining support at the country convention level.

Fraser, 54, is an announced candidate for election in November to the four-year balance of the late Hubert H. Humphrey's term. Muriel Humphrey, 66, who is serving as an appointed senator, has promised to announce early next month if she'll run. Sources close to her Washington office have predicted that Humphrey - the only woman in the Senate now - will declare that she wants to complete the term.

Minnesota has two Senate contests this year, and in the other one, yesterday's newspaper poll found Republican Rudy Boschwitz leading incumbent Democrat Wendell Anderson, 48-45 percent. Anderson has been criticized for arranging his own appointment to the Senate seat vacated by Vice President Mondale.