Hubert H. Humphrey (D-Minn.) has written to about 400 of his Minnesota Jewish supporters, sources of substantial Humphrey campaign help in the past, urging them to make "generous" contributions to Sen. Wendall Anderson (D-Minn.), who faces possible opposition for nomination next year.

In his Nov. 11 letter, which surfaced today, Humphrey stressed Anderson's support for Israel saying: "I know that he is as strong a friend of Israel as I am, and as effective in fighting for its survival. As a member of the powerful Senate Arms Services Committee, Wendy is particularly well-placed for this effort.

One of Anderson's Armed Services assignments is on the General Procurement Subcommittee.

Humphrey's letter described Anderson as "one of my closest friends and associates for many years and added, "I hope you will give generously - as generously as you have to me."

The letter preceded Egyptian President Anwar Sadat's mission to Israel, and Humphrey said in it, "Those of us who are strongly committed to the survival of the state of Israel have had much cause for concern in recent months."

"However well-intentioned, the initiatives of our government have not always been reassuring," he continued. "And the ever-growing economic and politicalpower of the OPEC (Organization of Petroleum Exporting Countries) nations raises great anxiety in my heart - and I know in yours, too - about what the future has in store for Israel."

Humphrey said Israel's security depends on the vigilance of members of Congress and added, "But we dare not lose the friends we now have in the Congress. Those who have demonstrated strong support for Israel's survival must be themselves supported, and returned to continue their work."

The letter was the first open showing that Humphrey would back Anderson in a nomination contest.

Anderson, a former Minnesota governor who in effect appointed himself to the Senate last December to fill Vice President Mondale's vacated seat, is already campaigning for a full term.

Rep. Donald M. Fraser (D-Minn.), has said he may oppose Anderson, and John Connolly, a St. Paul lawyer, has said he will seek the nomination, forcing a primary race if necessary.

On the Republican side, Minneapolis businessman Rudy Boschwitz has announced for the Senate seat.

Boschwitz, who is Jewish, has conducted money-raising efforts for Israel, and his backers hope he can lap some of the sources that traditionally have been Humphrey's Fraser has also had significant financial backing in that area.

Fraser has sent letters to 40,000 members of the Democratic Farmer Labour Party, as Democrats call it in Monnesota, saying the Senate seat should be regarded as "open" because Anderson has had neither Party endorsement nor voter approval in any election for the Senate. Fraser has previously said he will announce his decision on the Senate race early in December.