When it comes to presidential elections in Pennsylvania, Republicans are like Tantalus, the figure of Greek mythology. The man whose name lives on in the word tantalize was doomed to stand in a pool of water that he could never drink, while grabbing for fruit from a tree he could never reach - for eternity.

The Keystone State always looks winnable for Republicans, on paper, but in each of the last six presidential elections, it has slipped away. It's a strategic mirage.

In 1988, George H.W. Bush cracked the code, clawing his way to 50 percent of the vote in the Philadelphia media market, home to up to 42 percent of votes cast statewide. That enabled him to carry Pennsylvania with 52 percent.

Since then, of course, the GOP candidate has usually been buried in the city and its suburbs.

Last year, President Obama swept the city and its four suburban counties with 65 percent of the vote, while Mitt Romney had 55 percent in the rest of the state.

Despite this record of futility, the nonpartisan Cook Political Report crunched the numbers recently and concluded that Pennsylvania might just hold the biggest promise for the GOP. Not only is the state gettable, with the right candidate, but it has the biggest cache of electoral votes (20) among the few blue states that are within Republican reach.

Amy Walter, respected national editor of the Cook report, reached that finding using the publication's partisan voting index, or PVI, a measure of how each state voted in the last two presidential contests, compared with the nation as a whole. Wyoming's PVI, for instance, is R+22.

The Romney campaign expended time and money trying to win Nevada, with six electoral votes, while investing minimally in Pennsylvania. Yet the PVI scores indicate that Pennsylvania is less firmly in the Democratic column.

Nevada's PVI of D+2.07 makes it more Democratic than, say, Iowa (D+1.08), Minnesota (D+1.76), or Pennsylvania (D+1.15).

"The Romney campaign spent $8.9 million on broadcast TV in Nevada during the general election to get 46 percent of the vote," Walter wrote last week. "In Pennsylvania, the Romney campaign spent a paltry $2.4 million and got 47 percent. In other words, Team Romney spent four times as much in Nevada as they did in Pennsylvania, to get essentially the same percentage of the vote. Now, imagine that the money invested in Pennsylvania came earlier - and more intensely."

Pennsylvania Republicans tried hard to persuade the Romney high command at headquarters in Boston to make an effort in the state, but polls convinced those strategists that Nevada and Colorado, with nine electoral votes, seemed to be within closer reach.

To be sure, Obama and his Democratic allies would have countered hard in Pennsylvania - attack ads would have filled our airwaves - and it is possible that Romney, caricatured as a callous rich guy who didn't get the travails of the middle class, would not have been the most sellable candidate in the moderate suburbs of Philadelphia anyway.

"I'm not convinced that more money spent here would have made much of a difference," said pollster Christopher Borick, a professor of political science at Muhlenberg College.

"There was not a lot of movement in the polls in states where Romney played or didn't play," he said. "There simply weren't that many voters that were movable during the fall, and a big ad buy in Pennsylvania may have produced just as little as it did in Nevada for Romney."

Still, the Electoral College math is changing. For years Republicans had a built-in advantage, holding the partisan edge in states with more combined electoral votes than the Democrat-leaning states - so much so that pollsters and scholars spoke of a GOP "lock" on the Electoral College. Now the Democrats have a slight advantage.

Demographic shifts in states such as Nevada and Colorado, with their increasing numbers of Latino voters, mean Republicans will need to look for new targets in presidential elections, Borick said.

"You may indeed see Pennsylvania play a bigger role the next presidential race," he said - especially, say, if New Jersey's Gov. Christie, a Philadelphia market favorite, is on the ballot.

Could a Christie at the top of the 2016 ticket give Republicans a real shot at Pennsylvania?

It's a tantalizing thought.