"She really cares about moving this country forward," Rendell said.
Rendell, who was mayor of Philadelphia when Bill Clinton was president and general chairman of the Democratic National Committee in 2000, said Sen. Clinton's health care and economic development proposals dovetail policies that he has pursued as governor since 2003.
"I think the best guarantee that we'll (accomplish those goals) is having Hillary Clinton elected as president," he said.
Clinton was scheduled to be in New Jersey Wednesday evening to pick up another endorsement and attend two rallies.
Bergen County Democratic Party Chairman Joe Ferriero was to endorse the former first lady. She was scheduled to attend a rally in Hackensack followed by one in North Bergen specifically for Latino voters. A private fundraiser was also planned.
Clinton's appearance in New Jersey comes on the heels of two new state polls on the primary.
A Quinnipiac University poll released Wednesday showed the New York senator with a 16-point lead over Illinois Sen. Barack Obama. A Fairleigh Dickinson-PublicMind poll out Tuesday showed that Clinton and Obama topped voters' lists of candidates they'd be most likely to support in the general election.
However, Clinton was also shown to have three times as many negatives as Obama - 27 percent of those polled said they would not support her, compared with 9 percent who would not support Obama.
New Jersey's presidential primary is Feb. 5; Pennsylvania's is April 22.
Rendell talked with an Associated Press reporter before attending a groundbreaking ceremony at the Dickinson School of Law in Carlisle. A formal announcement was planned at an afternoon event in Philadelphia with Mayor Michael Nutter, who also has endorsed the senator from New York.
Sen. Clinton had endorsed Rendell in his 2006 re-election campaign, in which he handily defeated Republican challenger Lynn Swann, the former Pittsburgh Steelers star.
Pennsylvania's primary is April 22 - probably too late to affect the selection of the party's apparent presidential nominee - and Sen. Clinton's campaign hailed Rendell's early endorsement as a major coup.
"It's huge," said Philadelphia lawyer Mark Aronchick, a national fundraiser for the campaign, citing Rendell's popularity among Democrats nationally and his prodigious fundraising skills.
The Republican National Committee scoffed at the endorsement.
"Establishment liberals like Ed Rendell are eager to support Hillary Clinton's tired old brand of tax and spend government but they cannot repair her lost credibility," said RNC spokeswoman Blair Latoff.
Rendell appeared at a news conference in Los Angeles on Saturday with New York Mayor Michael Bloomberg, an independent, and California Gov. Arnold Schwarzenegger of California, a Republican, to announce a coalition that will push for investment in ports, highways and other infrastructure.
The event fueled speculation that Rendell might team up with Bloomberg if he launches an independent bid for president. Rendell acknowledged that his endorsement will halt those rumors.
"That's one good thing," he said. "It gets the attention off of that."