Pennsylvania stands squarely in the national spotlight as voters go to the polls Tuesday to elect a governor and U.S. senator, along with 19 U.S. House members.

With sunny skies predicted from Philadelphia to Erie, from Pittsburgh to Scranton, 7,858 polling places will be open for the state’s 8.5 million registered voters between 7 a.m. and 8 p.m.

In one of the country’s most closely watched Senate races, Republican former U.S. Rep. Pat Toomey of Allentown and Democratic Rep. Joe Sestak of Edgmont are battling to succeed Democrat Arlen Specter.

National Democrats, led by President Obama and former President Bill Clinton, have held rally after rally in hopes of pushing Sestak ahead of Toomey, who has led narrowly in polls. Monday evening, Michelle Obama appeared at the Perelman Quadrangle at the University of Pennsylvania, saying, “This election is about all we have left to do. We need you to get fired up.”

Another Democrat hoping to benefit is gubernatorial nominee Dan Onorato.

With no ceiling on campaign contributions for state offices, Onorato and Republican Tom Corbett, both from the Pittsburgh area, have spent a combined $50 million in their race to succeed Gov. Rendell. After two terms in office, Rendell cannot run again.

Corbett, the state attorney general, held a polling lead over Onorato, the Allegheny County executive. But Democratic leaders continued to show their belief that Onorato could pull it out by sending Obama and Clinton to campaign for him. A visit to McKeesport on Monday by Clinton marked his sixth appearance for Onorato.

The eyes of America are also on the state’s top House races.

With control of Congress up for grabs, seven of the state’s 19 House races have been targeted for TV ads, mailers, and phone calls by national partisans on each side.

These races include three in the Philadelphia suburbs.

In the Eighth District, Democratic incumbent Patrick Murphy faces a familiar foe: former U.S. Rep. Mike Fitzpatrick, whom Murphy narrowly ousted in 2006. The district includes all of Bucks County, a sliver of Montgomery County, and two wards in Northeast Philadelphia.

A bitter fight has taken hold in the Seventh Congressional District, a seat left vacant after Sestak decided to run for the Senate. Democratic State Rep. Bryan Lentz of Swarthmore faces Republican Patrick Meehan, a former U.S. attorney from Drexel Hill. The district includes most of Delaware County and parts of Montgomery and Chester Counties.

Also running in the Seventh District is Jim Schneller, 55, of Wayne, a conservative third-party candidate.

Republican U.S. Rep. Jim Gerlach of Chester Springs is running for his fifth term in the Sixth District. He faces Manan Trivedi, a Reading physician and Iraq war veteran. The heavily gerrymandered district includes Montgomery County, Chester County, Berks County, and a piece of Lehigh County.

Southeastern Pennsylvania TV viewers could be pardoned if they thought either U.S. Rep. John Adler or former Eagles tackle John Runyan was a member of their House delegation. The rivals from New Jersey’s Third District have bombarded Philadelphia airwaves with their ads.

The candidates for the U.S. Senate from Delaware also have made themselves well known to Philadelphia-area TV viewers. That race, also one of the nation’s mostly closely watched, pits Democrat Chris Coons against tea party Republican Christine O’Donnell.

Candidates of all stripes made final appeals to voters Monday.

Corbett and Toomey barnstormed the state together by jet. During their first stop, at the Four Points Sheraton in Northeast Philadelphia, Corbett urged 150 supporters to turn out to vote so that, as governor, he could cut a state budget he said had been bloated by eight years of excess.

“Don’t do it for me; don’t do it for yourselves,” he said. “Do it for the children of Pennsylvania. . . . This is a generational election.”

Toomey warned against Republican overconfidence.

“Elections don’t get won by accident,” he said. “The other side has not given up . . . they’re fighting as hard as they can.”

The candidates, accompanied by Jim Cawley of Bucks County, the GOP lieutenant governor nominee, boarded a plane at Northeast Philadelphia Airport and headed off to rallies in Wilkes-Barre/Scranton, Harrisburg, Johnstown, Erie, Pittsburgh, and Allentown.

In her appearance at Penn, Michelle Obama sought to rally support for Democratic candidates, including Sestak, Onorato, and Allyson Y. Schwartz and Chaka Fattah, who are running for reelection to Congress. Also on stage were Rendell, Mayor Nutter, and U.S. Sen. Bob Casey, who is up for reelection in 2012.

“We’ve come too far to stop putting the American dream . . . back within reach for every American,” Obama said.

Sestak’s day included a frenetic, 12-event schedule around Philadelphia and its inner suburbs, beginning with a 5:30 a.m. round of handshaking at 30th Street Station.

By 11 a.m., Sestak was in his campaign’s storefront office in Ardmore, exhorting 25 volunteers who were organizing phone-banking and canvassing shifts.

“I want to serve because I believe people want someone in Washington who recognizes it’s all about the middle class and those who want a fair opportunity,” Sestak said. “It’s been a long, hard fight, and I’ve enjoyed almost every moment of it.”

Specter, 80, lost to Sestak in the May primary and will leave office after 30 years in January.

Onorato also started the day greeting commuters, at the Market East station. He moved on to McKeesport, where he joined Clinton at a Pennsylvania State University campus. He was then off to Harrisburg for a rally with Martin Luther King 3d, oldest son of slain civil-rights leader the Rev. Dr. Martin Luther King Jr.

“It’s about turnout now,” he said of the election in McKeesport. “It’s about getting out tomorrow. It’s right there in front of us.”

Election officials and district attorneys in the state’s 67 counties will be alert for illegal activity at polls, including fraud and attempts to block people from voting.

Philadelphia District Attorney Seth Williams said: “We encourage all registered voters in the city to get out and exercise the privilege of casting their votes. That being said, we want to stress that we will go after any criminal activity and prosecute it to the fullest extent of the law.”

The Committee of Seventy, the city’s watchdog group, said a small number of Philadelphia voters who have sent in absentee ballots had reported that the ballots came back to them in the mail instead of being forwarded to the election office.

The deadline for absentee ballots to be submitted was Friday. The committee urged the city to extend the deadline, but it quoted city officials as saying they would handle the complaints case-by-case.

For Election Day, the committee said, it had trained 550 nonpartisan volunteers to monitor polling places in every city ward.

The monitoring will focused on the 172d state House District in the Northeast, where Republican State Rep. John M. Perzel, the former speaker of the House, is in a contentious battle with Democrat Kevin Boyle.