Of the many high-profile contests on tap today in four states, none will be more closely scrutinized than the special election in the southwestern Pennsylvania district that was long the bailiwick of the late Democratic Rep. John Murtha.

The Pennsylvania special is the only contest that will change the partisan composition of Congress, and as such the outcome will be heavily spun by both parties and analyzed at length about what it portends — if anything — for the November elections.

Election eve polls show businessman Tim Burns (R) and ex-Murtha aide Mark Critz (D) are essentially in a dead heat, so officials in both parties were managing expectations in the final hours leading to today’s vote because they don’t know yet if they’ll be crowing about a victory or explaining away a loss.

Democratic officials are playing the expectations game by noting that Obama is less popular in the 12th district than in most competitive districts and that it’s the only district in the nation to have voted for Sen. John Kerry (D-Mass.) in the 2004 presidential election and then for Sen. John McCain (R-Ariz.) in 2008.

“Despite all the NRCC hype and the million-plus spent by Republicans and their shadowy outside groups, we expect this race to go right down to the wire,” said Ryan Rudominer, a spokesman for the Democratic Congressional Campaign Committee.

If Republicans can’t win Murtha’s district, Democrats say, then how can they expect to capture the 40 Democratic-held seats they need to win the majority in what GOP officials anticipate will be a great year for the party?

“Right now, I think the pressure is more on the Republican side because they’ve been trying to send a message that there’s this Republican wave election coming in the fall,” said Mark Nevins, a Pennsylvania-based Democratic consultant and a former DCCC aide. “They need to win the special elections to lend credibility to that assertion on their part. If they don’t win the specials, it’s hard to make the argument that there’s a wave election coming.”

Republicans are playing the expectations game by noting that the district is historically Democratic and that Gov. Ed Rendell (D) purposely scheduled the special election to coincide with the regular statewide primary — the idea being that intensely competitive Democratic Senate and gubernatorial primaries would boost turnout in the 12th district.

“In a district that has a 2-to-1 Democrat registration advantage, the fact that we are running neck and neck demonstrates just how frustrated the American people are with government takeovers and bailouts,” National Republican Congressional Committee spokesman Tory Mazzola said.

Several political analysts cautioned against reading too much into the result in Pennsylvania’s 12th. They said special elections have limited predictive value, and many factors can influence a nationwide vote that is still six months away.

“I think they definitely get overanalyzed, because I’m not sure you can make any predictions about specials and what is going to happen in the fall,” said Carl Forti, a Republican consultant and former NRCC official. “The one thing we know of right now is that there is a lot of voter anger and angst out there, and we’ve seen that reflected in different races around the country. How that manifests itself in this particular race remains to be seen.”

Former Rep. Martin Frost (D-Texas) noted that his party lost all but one contested special election when he chaired the DCCC in the 1996 and 1998 cycles, yet House Democrats still made gains in the November elections.

For that reason, Frost said, “I wouldn’t pay too much attention to the results in the special election in Murtha’s seat, although the Republicans certainly will hype it should they happen to win.”

Though special elections may not necessarily presage November outcomes, a win in Pennsylvania would yield some psychological benefits to the winning party, including favorable talking points for the fall and revved-up donors.

Former Rep. Tom Davis (Va.), a former NRCC chairman, said that “we may read too much into the returns one way or the other, but you want to win. You don’t want to come close on this one because it’s a momentum-builder.”

Noting the narrow Republican losses in special elections last year in New York’s 20th and 23rd districts, Davis said that “Republicans need a win” and that Pennsylvania’s 12th is the kind of district “that is ripe to send a message” by electing a Republican.

Frank Donatelli, the chairman of GOPAC and a former Reagan administration aide, said that a Burns victory would have an “explosive” impact, and that result or even a narrow Burns loss would impel Democrats to distance themselves from the White House.

“The immediate repercussion would be a moving away by a lot of moderate Democrats from the Obama agenda,” Donatelli said. “They’ve had to cast a lot of tough votes to begin with — stimulus, health care and so forth — and if Republicans were to win that seat or to come close, I think you would see a lot of Democrats who represent moderate districts not necessarily voting the way the president wants.”

A Critz win could give nervous Democrats a confidence boost as they prepare to defend their House majority — and illustrate that the GOP campaign to win a House majority is easier said than done.

Frost said a Democratic win “would send a good message that while the House is competitive, that the Democrats can still win races in the House and that it’s not going to be easy for the Republicans to take the number of seats they need to win back the majority.”