Mayor Bloomberg is taking his already enthusiastic support for Senator Lieberman to the next level: He is planning to hold a fund-raiser the week before the general election.

The fund-raiser — to be held on November 1 at Mr. Bloomberg’s Upper East Side townhouse — comes with Mr. Lieberman attempting to win back his Connecticut seat as an Independent after losing the Democratic primary to anti-war challenger Ned Lamont.

Mr. Bloomberg, a lifelong Democrat who became a Republican just before running for mayor, has repeatedly said Mr. Lieberman’s willingness to put his views before his party’s is something that’s sorely lacking in Washington.

While high-profile Democrats have ditched Mr. Lieberman, Mr. Bloomberg and other Republicans have come to his defense. Mr. Lieberman has even hired a former Bloomberg campaign consultant, Josh Isay.

Although the two politicians are cut from the same political cloth, the move could also help the mayor if he decides to run for president in 2008 as an Independent.

Mr. Bloomberg has criticized elected officials who abandoned Mr. Lieberman after his support of the Iraq war torpedoed his Democratic bid for reelection and has told reporters that the country needs more “nonpartisan elected officials who think doing the right thing for the public is more important than supporting some party.”

A New York City supermarket magnate who is helping with the Lieberman fund-raiser, John Catsimatidis, said the event is in the planning stages. He would not comment on the location, but other sources said it would be in the mayor’s home — a venue he has used in the past for fund-raisers and city events.

Mr. Bloomberg has vowed to pitch in and help Mr. Lieberman if asked by the senator, but this would be his first major contribution to the campaign.

Mr. Catsimatidis — a lifetime Democrat and supporter of President Clinton — is said to be considering a run for mayor in 2009 as a Republican. He said he is calling friends from both major political parties to make donations to the Connecticut legislator.

A spokeswoman for the Lieberman campaign, Tammy Sun, confirmed that Mr. Bloomberg would be involved in a fund-raiser but would not provide any additional details. She said the planning was in the “preliminary stages.”

A spokesman for the mayor, Stuart Loeser, would not comment yesterday on the fund-raiser, but reiterated the mayor’s past support for the senator.

“The mayor’s said he’ll do whatever Senator Lieberman wants,” he said. “They don’t see eye to eye on all issues, but the mayor respects elected officials who speak their mind and stick to their convictions, and do what they think is right for their constituents — not just what special interests and party leaders think they should do.”

It is unclear whether invitations for the November event have been sent out or if a suggested contribution has been decided. The additional funds could help Mr. Lieberman with funding television commercials and other campaign promotions in the final week of the campaign before the November 7 election.