(This Nov. 5 story has been corrected to fix the age of supporter in paragraph 10)
By Jarrett Renshaw and Gram Slattery
PHILADELPHIA (Reuters) - The biggest names in Democratic and Republican politics -- Joe Biden, Barack Obama and Donald Trump -- head to Pennsylvania on Saturday hoping to tip the balance in a closely contested midterm race that could determine control of the U.S. Senate.
Former President Obama caps a five-state tour aimed at stemming his party's losses in Tuesday's congressional elections with appearances in Pittsburgh alongside Democratic Senate nominee John Fetterman before heading to Philadelphia, where he will take the stage at Temple University with President Biden.
Former President Trump, meanwhile, will be ginning up support for his hand-picked Republican Senate nominee, celebrity doctor Mehmet Oz, and Republican gubernatorial nominee Doug Mastriano at a rally in Latrobe, southeast of Pittsburgh.
As he has in a steady stream of rallies since leaving office, Trump is also working to maintain his own profile as he contemplates launching a third run for the White House after the midterms, according to advisers.
That could set the stage for a Biden-Trump rematch, though some Democrats say heavy losses for Biden's party on Tuesday could increase pressure on the president to step aside and let someone else carry the party's mantle in 2024.
The Fetterman-Oz Senate race is one of three critical contests, along with Georgia and Nevada, that will determine whether Democrats hold onto their razor-thin majority in the Senate, and with it the power to confirm Biden's nominees to posts ranging from his Cabinet to the Supreme Court.
Nonpartisan election forecasters and polls show Republicans are heavy favorites to win control of the House of Representatives, with the Senate a toss-up. Control of even one of those chambers would give Republicans the power to block Biden's legislative agenda and launch potentially damaging investigations.
Both parties have lavished attention on Pennsylvania both because of the strategic importance of the race and because of its voters' history of swinging from one party to the other in the past four presidential elections.
Volunteers are also out across the state.
Susan Mast, a 59-year-old teacher from Lancaster, has spent several hours each day for the last four weeks knocking on doors trying to recruit voters for Fetterman and other Democratic candidates.
"Democracy is at stake. Women's rights are at stake," Mast said.
Fetterman, the state's lieutenant governor, held a commanding lead in the race throughout the summer, which Oz has whittled away in the last two months.
Some factors may be local: a stroke this spring forced Fetterman to scale back his campaign schedule and has affected his speech. At a debate last month, he often stumbled over his words, in a performance even allies privately described as shaky.
But Oz's gains also reflect a nationwide momentum shift in favor of Republicans, as voters' focus on inflation and crime has proven more durable than concerns about abortion. Democrats' early lead in several other Senate races, including the contests in Georgia and Nevada, have also shrunk or evaporated completely in recent weeks.
Also playing against Democrats is Biden's unpopularity. Only 40% of Americans approve of the president's job performance, according to a Reuters/Ipsos opinion poll completed on Tuesday, which has led Biden to hold back from campaigning in some key states.
Obama has by contrast been tearing up the campaign trail over the past week in the nation's most fraught battlegrounds -- including Georgia, Michigan, Wisconsin and Nevada.
One bright spot for Democrats is Pennsylvania's gubernatorial race, in which Democratic Attorney General Josh Shapiro holds a commanding lead against Mastriano, a Republican state senator whose far-right stances have failed to connect with voters.
Mastriano, who would get to appoint Pennsylvania's secretary of state and exert heavy influence over the conduct of elections were he to win, introduced a failed resolution after Trump lost Pennsylvania in 2020, claiming falsely it was the Republican-controlled legislature that had the power to determine which candidate received the state's presidential electors' votes.
Mastriano has also said that if elected he would ban abortions after six weeks of pregnancy, a position that stands out among Republicans running in swing states, many of whom have sought a middle ground on the issue.
(Reporting by Jarrett Renshaw in Philadelphia and Gram Slattery in Washington; Editing by Scott Malone and Daniel Wallis)