Some of Vice President Mondale's best known home-state backers today launched a rear-guard action against independent presidential candidate John B. Anderson.

The effort was directed at Democratic liberals, who Mondale allies fear will vote for Anderson.

Hours before the Illinois congressman was to arrive here for a campaign stop, state Attorney General Warren Spannaus, cochairman of the Carter-Mondale campaign here, called a news conference.

Spannaus didn't condemn Anderson the man. He praised him. "He's run a good race," he said in an interview. "He's a good man. There are a lot of things I respect about John Anderson."

But Spannaus, the state's highest ranking elected Democratic-Farm-Labor Party official, said Anderson "is no longer a factor in the race" and voters "should not waste their vote on him."

The reason for concern among Mondale supporters here is a poll published in the Minneapolis Tribune last Sunday showing President Carter leading Reagan by only 34 to 30 percent in a threw-way race including Anderson. In a two-man race, Carter would win by 45 to 34 percent, the poll said.

Anderson drew 18 percent in the poll, a sharp decline from his fortunes of last July when he and Carter were tied at 23 percent.

To preserve the principles Anderson has campaigned for, Spannaus said, he should take the $2 million he has budgeted for television commercials and donate the money to congressional candidates who share his views.

Anderson, at his own news conference, dismissed Spannaus' suggestion as a "desperate cry of a campaign in deep trouble . . . It's nothing short of preposterous. Not only do we have a failed presidency, we have a failed candidacy."

Spannaus also released today a letter that is being sent to liberals in the state. "We acknowledge support for John Anderson here in Minnesota. We compliment him on his persistence. But working people, family farmers, seniors and the handicapped need not suffer the test of four years under a New Right Reagan administration," the letter said.

"There may be no perfect choices in 1980, but we believe continuation of our work on behalf of progressive programs under a Carter-Mondale administration is the best hope we have of securing a better future," it added. The letter was signed by some of the state's best-known liberals, including Dave Rose, president of the AFL-CIO; Mike Freeman, son of former Minnesota governor and agriculture secretary Orval Freeman and Cy Carpenter, president of the state Farmers Union.