HARRISBURG -- For months Gov. Ed Rendell, a former chairman of the Democratic National Committee, has kept mum about which of the Democratic presidential contenders he favors in 2008.

That changed yesterday, as he announced, first in Carlisle, Cumberland County, and then in Philadelphia, that he's backing New York Sen. Hillary Rodham Clinton.

"She really cares about moving this country forward," he told The Associated Press, adding that her performance as a senator has been "spectacular."

So now an important question arises: How much does his endorsement matter?

The answer to that, said political experts, depends on whether the Pennsylvania presidential primary, which won't be held until April 22, still has any relevance to the choice of the Democratic candidate. With 22 primaries on Super Tuesday, Feb. 5, the Democratic nominee will have the nod all wrapped up by then.

"It's clearly positive news for Mrs. Clinton," said Christopher Borick, a political science professor at Muhlenberg College in Allentown. "And yet, how much it means in terms of actual impact on changing voters' minds is probably fairly limited. Endorsements, in general, are often overrated in terms of the impact on voters' preferences."

In the states with either Democratic or GOP primaries or caucuses on Super Tuesday, a governor's endorsement can be important, other observers said. It can lend prestige, help with grass-roots organizing to get out the vote and boost fund-raising efforts.

So far, 24 governors, including Mr. Rendell, have endorsed a presidential candidate. Among Democrats, Mrs. Clinton leads with 10 endorsements, including those of Mr. Rendell, New York Gov. Eliot Spitzer and Ohio Gov. Ted Strickland. Her main challenger, Illinois Sen. Barack Obama, has the backing of Massachusetts Gov. Deval Patrick and Illinois Gov. Rod Blagojevich.

Thomas Baldino, a political science professor at Wilkes University in Wilkes-Barre, said he wasn't surprised about Mr. Rendell's endorsement, since he was Democratic Party boss in 2000, the last year of Bill Clinton's presidency. Also, Mr. Rendell had gotten to know the Clintons during the 1990s, when Mr. Rendell was mayor of Philadelphia for eight years and Mrs. Clinton was first lady.

Mr. Rendell said one reason he favors Mrs. Clinton is her advocacy of health care plans. Mr. Rendell is trying hard to get his health insurance plan called Cover All Pennsylvanians enacted in the Legislature.

Some political experts cast doubt on the importance of the Pennsylvania primary, since the Democratic and Republican races might be locked up after Super Tuesday.

Mr. Baldino said he's still hoping "the Pennsylvania primary may be relevant" in determining the presidential nominees.

If former North Carolina Sen. John Edwards, who is currently running third behind Mrs. Clinton and Mr. Obama, "can hang in there, and if the race isn't locked up" in February or March, Mr. Baldino said, the Pennsylvania primary, and Mr. Rendell's endorsement of Mrs. Clinton, could be meaningful.

Mr. Rendell remains very popular in voter-rich areas of southeastern Pennsylvania, including Philadelphia and several populous suburban counties, and his support for Mrs. Clinton could help her there. Mr. Rendell doesn't have the same kind of clout in Central Pennsylvania and many parts of Western Pennsylvania.

Mr. Rendell's endorsement of Mrs. Clinton might also be a way of wangling a federal appointment in the future. He has a strong interest in biofuels and reducing American dependence on foreign oil, and has admitted interest in someday becoming federal energy secretary if a Democrat is elected president in November. But he insists he'll finish out his current four-year term as governor, which doesn't end until January 2011.