Businessman Tom Knox will abandon his plans to run for mayor and announce his support for Mayor Nutter today, a source familiar with his intentions told me last night.

Knox will join former Gov. Ed Rendell at a morning news conference at the Hyatt at the Bellevue, where both will endorse the incumbent in the 2011 mayor's race.

Knox's reversal removes the most potent threat to Nutter's re-election. Knox had said he was prepared to spend millions of his own funds to defeat Nutter, either in the Democratic primary or as an independent in November.

The source who spoke of Knox's plans was unable to say why Knox had decided to back out and support Nutter, whom Knox criticized as having accomplished little in interview last year.

(UPDATE : The Nutter re-election camaign now confirms that Knox will join Rendell in endorsing Nutter.)

Knox commissioned a poll last week to assess his chances, and he told me after an early look at the findings that a run in the general election seemed like the most attractive alternative.

He said his wife didn't want him to run, but added, "my wife is a real trooper and if I really wanted to do it, she'd be there with me. And I've always wanted to do it."

Reached yesterday, Knox declilned to discuss his plans, but said he would make his intentions known today.

Knox finished second to Nutter in the 2007 Democratic mayoral primary, when there were five well-known candidates in the field. Despite spending $13 million on that contest, a Franklin & Marshall poll last month found 57 percent of voters said they hadn't heard of Knox.

Knox always said he would quickly become re-acquainted with voters when he began buying TV ads.

Rendell's endorsement will be a second dose of good news for Nutter, since Rendell remains popular in the city. Rendell and Nutter had a sometimes contentious relationship in the 1990's, when Rendell was mayor and Nutter was in City Council.

Rendell did not endorse a candidate in the 2007 mayoral primary, but supported Nutter in the general election.

The only candidates who have announced plans to challenge Nutter so far are Democrat Milton Street and Republican John Featherman.