Scores of politicians mixed with tens of thousands of union partisans Monday in the Labor Day rite that marks the unofficial kickoff of the fall political season.

Across from the Civic Arena, near the front of this showcase for a once-reliable Democratic Party constituency, was Attorney General Tom Corbett, the GOP candidate for governor, marching with members of the Fraternal Order of Police. In a year in which Republicans hope to make major gains, was the high-profile position of the GOP candidate an omen?

"No question, it's going to be a tough year," said Michael Lamb, the Pittsburgh controller. "But there's eight weeks to go; today's the day when this race really starts."

History, a bad economy and reams of polling data are foreboding for the party that captured the White House in a landslide just two years ago. If there was one bit of good news for Democrats on Monday, it was that the election would not take place today.

"It's a scary year for Democrats," said state Rep. Nick Kotik, D-Robinson, as he strode toward Grant Street. "It's not just what's happening in Washington. [Gov. Ed] Rendell is an anchor in my district. That's going to hurt Onorato as well," he said, referring to the Mr. Corbett's rival, Allegheny County Executive Dan Onorato.

A few blocks farther back, a smiling Mr. Onorato greeted well-wishers along the route and professed confidence in his chances. Asked how he could restore the luster to his party's battered brand, he said, "It's not about brands, it's about policies. As people focus on policies, you'll see the movement."

Mr. Corbett, buoyed by consistent polling leads, maintained that the momentum would remain with his party's policies.

"We've got to get the spending under control -- you've heard me say that all over the state. Then we can cut taxes," he said.

Mr. Onorato was marching with AFSCME, the government workers' union with whom he occasionally has clashed on county personnel issues. Henry Hurt, the president of AFSCME Local 2719, said that while Mr. Corbett had some support within his membership, the union was firmly behind the Democrat.

"Initially, there was some consideration for Corbett -- he represents some of our members and the feeling was, he treats them well, but he made some decisions, some policy decisions, and we made our decision," Mr. Hurt said, alluding to the Republican's calls for deep cuts in state spending.

The Onorato-Corbett contest is one of 37 gubernatorial races across the country this November -- 19 now held by Democrats and 18 by Republicans.

Thirty-seven U.S. Senate seats also will be contested amid GOP hopes for a gain of 10, which would give them control of the chamber. Along Grant Street and down the Boulevard of the Allies, U.S. Rep. Joe Sestak was doing his energetic best to counter that anticipated trend. The Democratic Senate nominee jogged back and forth along the parade route, grasping hope with every handshake.

"The leadership of our party sometimes was more concerned with politics than policy," he said, endorsing the widespread perception of an anti-incumbent mood among voters.

Discussing a months-long pattern -- as measured by polls and voter turnout in primaries -- of greater political intensity among Republicans than Democrats, he said, "You don't just sit there and wring your hands -- 'Oh, we have an enthusiasm gap.' You go out there and do something about it."

Mr. Sestak faces former U.S. Rep. Pat Toomey in the Senate race. Both Mr. Sestak and Mr. Onorato have trailed their opponents in every post-primary poll. Jim Burn, the state Democratic Party chairman, insisted that party and union turnout efforts would counter that intensity gap.

"It's not like two years ago where you'd put out an Obama sign and 500 people would show up," he said. "But those people are still out there and it's our job to reach them, and that's what we're doing with our coordinated campaign."

President Barack Obama spent part of his Labor Day in Milwaukee, proposing in a speech to that city's Laborfest that the government could boost employment by spending $50 billion on transportation infrastructure. On Wednesday, he will be in the Cleveland area, discussing tax breaks for small business in a state where his party's candidates for governor and U.S. senator entered September with double-digit polling deficits.

Analysts of both parties see the likelihood for significant GOP gains in the U.S. House, where a net swing of 39 seats could place the speaker's gavel in the hands of Rep. John Boehner, R-Ohio. At least 10 of Pennsylvania's 19 congressional districts are among those considered to be genuinely up for grabs in the coming weeks. Following the national pattern, most of those competitive seats are held by Democrats, a party that saw major congressional gains in each of the past two elections here and across the nation. In Western Pennsylvania, Rep. Mark Critz faces a rematch with Tim Burns, the Republican he defeated in a spring special election for the 12th District seat long held by the late Rep. John P. Murtha. In the 4th District, Rep. Jason Altmire, a member of the 2006 class that gave Democrats control of the House, faces a challenge from lawyer Keith Rothfus. To the north, freshman Democrat Kathy Dahlkemper is working to fend off a 3rd District challenge from businessman Mike Kelly. In the east, from the Philadelphia suburbs, through the Lehigh Valley and on to the New York border, at least five Democratic incumbents are facing tough GOP challengers.

That region also includes two of the relatively few Republican seats where Democrats hope to counter the anticipated GOP tide. Rep. Charles Dent, R-Allentown, faces a well-funded challenge from Bethlehem Mayor John Callahan in the state's 15th District. In the nearby 6th District, Rep. Jim Gerlach is opposed by Manan Trivedi, a physician who was the upset winner of a tough Democratic primary.

"[Rep.] Pat Murphy has a tough race ... [Rep.] Paul Kanjorski has a tough race, but we feel we have a chance to pick up the 6th and the 15th," Rick Bloomingdale, president of the state AFL-CIO, said as he watched his union brethren assemble for their march. "Those are all districts with high union density."

Pennsylvania is also one of the states where control of the Legislature hangs in the November balance. The state Senate remains firmly in control of the Republicans. But the state House, which, like the U.S. House, reverted to Democratic control in 2006, is very much in play. If Republicans were able to reverse the Democrats' narrow majority, and if Mr. Corbett were able to preserve his apparent lead against Mr. Onorato, that would put the GOP in control of the redistricting process that will chart the state's congressional districts for the next decade.

Republicans controlled Pennsylvania's last redistricting a decade ago. One of the beneficiaries was Rep. Tim Murphy, R-Upper St. Clair, who moved into a seat carefully charted to enhance GOP chances. Since then, he's fended off a succession of Democratic challenges. Now he is running against Dan Connolly of South Park. As he and several aides stowed campaign signs in a car parked near the end of the parade route, he contrasted this year's mood with the Democratic waves of 2006 and 2008.

"Labor Days are always fascinating," the Republican said. "The wind is more at our backs this year."

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