More than 40 law school deans, in a highly unusual move, have signed a letter to the Senate opposing President Reagan's nomination of Daniel A. Manion to be a federal appeals court judge, knowledgeable sources said yesterday.

Deans James Vorenberg of Harvard, Guido Calabresi of Yale, Robert Pitofsky of Georgetown, Robert Bennett of Northwestern and Norman Redlich of New York University are among those who signed the letter, which is still being circulated, the sources said.

Saying that Manion has "virtually no experience with major federal or other litigation," the letter states: "We urge that the Senate decline to consent to this nomination on the grounds that Mr. Manion is not qualified to serve on that court," the 7th Circuit Court of Appeals in Chicago.

Two other deans, Robert Mundheim of the University of Pennsylvania Law School and Gerhard Casper of the University of Chicago Law School, have sent a similar letter to Senate leaders.

Meanwhile, Senate Democrats have opened a new line of assault on Manion, accusing him in a report of "memory lapses and evasions" in testimony that they say was meant to hide his extreme views.

But Republican supporters charged in a separate report that the opposition to Manion "is rooted in ideology" and based on "an incomplete or distorted representation of the facts."

These two Judiciary Committee reports reflect each side's strategy in preparing for the Senate debate -- which could begin tomorrow -- on President Reagan's nominee. Reagan raised the political stakes in the confirmation fight in his Saturday radio address, giving Manion a strong personal endorsement and asserting that the real objection to Manion is that he "doesn't conform to the liberal ideology of some senators."

Both sides say the fight over Manion, 44, a former Indiana state senator and son of the late conservative broadcaster and John Birch Society leader Clarence Manion, is too close to call. The Democrats, who would like to drag out the battle, are unlikely to agree to a request yesterday by Majority Leader Robert J. Dole (R-Kan.) to limit debate to eight hours.

"We either have the votes or we don't have the votes," Dole said. "We might as well find out."

Sen. Dan Quayle (R-Ind.), who went to Indiana University Law School with Manion, is lobbying colleagues on Manion's behalf and recently helped arrange meetings between the nominee and a half-dozen undecided senators, mostly Democrats. Democratic opponents, led by Sen. Joseph R. Biden Jr. (Del.), estimate they will hold 45 of their party's 47 members and that they must pick up six Republican votes to defeat Manion.

At least two Republicans -- Arlen Specter (Pa.) and Charles McC. Mathias Jr. (Md.) -- are expected to vote against Manion, as they did last month when the Judiciary Committee failed to approve his nomination on a 9-to-9 vote. The panel then reported the nomination without a recommendation.

The report signed by Biden and four other Democrats attempts to defuse the charge that the case against Manion rests on his conservative ideology. Instead, it says Manion has not been honest about many of his past remarks, such as his call to strip the Supreme Court of the power to review obscenity cases.

At his confirmation hearings, the report says, "Mr. Manion either did not remember or attempted to evade responsibility for numerous earlier statements and beliefs he had held during his public and professional career," raising "serious doubts" about his "credibility."

For example, when asked about his past praise for the John Birch Society, Manion testified that "I could not tell you what the policies of the John Birch Society are." The report calls this "difficult to believe."

The Democrats also say that the South Bend attorney, who has never published a legal article or been the lead attorney in a federal case, is "an undistinguished lawyer" who is not qualified for the appeals court.

As evidence, the report cites some of the cases that Manion picked as among the 10 "most significant" he has handled: "A) Defense of a claim that Mr. Manion's client had improperly repaired a Volkswagon Rabbit, resulting in poor gas mileage and overall inferior performance; B) Two land condemnation actions, in one of which the issue was whether the injury caused by the erection of a fence totaled $8,000, as claimed by the private landowner, or $193 as asserted by the state; C) Defense of a claim that a truck driver was liable for damage to his cargo when his truck hit a 'dip' in the road . . . . "

But the Republican report says it is "misleading" to disparage Manion's cases simply because they may involve small amounts of money. It says his work in state courts "should not be brushed aside" and that to insist that every potential judge have argued a constitutional case "would exclude most small-town, small-firm practitioners."

Manion wrote about 20 appeals briefs as a deputy attorney general in Indiana, the GOP report says, and "has personally tried about 35 cases before a jury" and participated in complex federal securities cases.

The report cites the Chicago Council of Lawyers, which opposes Manion, for acknowledging his "high reputation for integrity, conscientiousness and fairness," and notes endorsements from Notre Dame University President Theodore M. Hesburgh and other liberals in Indiana. It says critics have tried to tar Manion "by association" with extreme views that he does not hold.

Responding to a Democratic charge that Manion's legal briefs reveal "a pervasive lack of technical ability," the Republican report says Manion had acknowledged that the briefs were unedited and contained typographical errors.