WASHINGTON — John McCain's efforts to snare Pennsylvania appear to be faltering despite a substantial commitment of his time, leaving him with a narrower path to the magic number of 270 electoral votes needed to win the presidency.

McCain is targeting Pennsylvania in hopes of winning at least one state that voted for Democrat John Kerry in the 2004 election. With 21 electoral votes, a victory in Pennsylvania could offset possible losses in smaller states captured by President Bush in the last contest.

Yet by any number of measures, McCain's prospects are dimming. An aggregate of public polls shows Barack Obama with a double-digit lead in Pennsylvania. Registered Democrats outnumber Republicans by more than 1.1 million, about twice the gap in 2004, state figures show.

McCain aides insist that they can win Pennsylvania. Recognizing the stakes, McCain is spending much of the remaining campaign time traversing the state.

Depriving Obama of a win here is essential for McCain. If Obama holds Pennsylvania, he could clinch the presidency by putting together various combinations of states that voted Republican four years ago but are now tilting Democratic: Colorado, New Mexico, Iowa, Virginia and North Carolina, among them.

A look at McCain's campaign schedule attests to his predicament. He is playing defense, trying to hold Republican territory. Apart from Pennsylvania, he has campaigned since Friday in Florida, North Carolina, Virginia, Ohio and Missouri — all states that backed Bush four years ago. Polls show Obama leading or nearly tied in each of them.

Pennsylvania Gov. Ed Rendell, a Democrat, said yesterday: "Pennsylvania is essential to their (McCain's) victory plan, though it's a long shot. If you assume Iowa is gone and New Mexico is gone and Virginia is gone, they have to win a substantial blue state. And we're the best choice out of a lot of bad choices."

But Rendell added that an Obama victory is no sure thing. To give Obama a lift, Rendell said he has pressed for Bill and Hillary Rodham Clinton, popular figures in Pennsylvania, to make return visits before Election Day.

Yesterday marked the 18th day McCain has visited Pennsylvania in the general election contest. He planned three rallies in the state, crossing east to west from Philadelphia to Harrisburg to Pittsburgh. His wife, Cindy, made four stops in the Philadelphia area and in York on Monday, and his running mate, Sarah Palin, appeared in Lancaster over the weekend.

Richard Thornburgh, a former Republican governor of Pennsylvania, said that barring a "November surprise" the odds of a McCain victory in the state are small. "That's the only thing that could turn it around, and I don't know what that could be," said Thornburgh, who was attorney general under presidents Ronald Reagan and George H.W. Bush.

Undaunted, McCain aides said they won't give up on the state. Mike Duhaime, political director of the McCain campaign, insisted yesterday that "we feel we're going to be successful" in Pennsylvania, calling McCain the strongest Republican in the state since Reagan.