They have been shadowboxing at separate campaign stops across the United States for weeks but the Democratic and Republican leaders find themselves on the same battlefield Saturday as they make closing pitches in Pennsylvania for next week's midterm election.

President Joe Biden will rally alongside his old boss Barack Obama as the Democrats deploy their big guns to build the energy they hope will spread nationwide and reverse the late rightward-shift in polling.

And in a split-screen preview of a potential rematch of the 2020 presidential contest, the midwestern state is also playing host to Biden's predecessor and bitter political rival Donald Trump.

Obama -- still the party's most bankable star six years after leaving the White House -- begins the day in Pittsburgh with Democratic candidate John Fetterman, who is in a dead heat against Republican TV medic Mehmet Oz in their crucial Senate race.

Biden and Obama then appear in Philadelphia, the historic cradle of US independence where the 44th and 46th presidents will woo voters from the suburbs that make for a crucial base of Democratic support.

The Keystone State backed Trump over Hillary Clinton for president in 2016 but preferred Biden to Trump in 2020.

Strategists from both parties believe the side that wins the post vacated by retiring Republican Pat Toomey will hold the majority in the upper chamber of Congress next year.

Fetterman and Oz sparred for an hour in state capital Harrisburg 10 days ago, with Fetterman still struggling with communication issues after a stroke in May upended his campaign.

'Chipping away'

"The month-to-month shifts in support for Oz are not statistically significant," said Patrick Murray, director of the independent Monmouth University Polling Institute.

"The overall trend suggests he has been chipping away with some voters who have not been completely comfortable with him, but that mainly happened prior to the debate."

Just a few miles east of Pittsburgh in Latrobe, Trump -- the one-term 45th president with ambitions to return as the 47th -- will seek to firm up support in a region that delivered him big margins in 2016 and 2020.

Pennsylvania is seen as a must-win not just for control of the Senate, but also for the balance of power among the country's 50 state governors, influential officials that weigh in on most aspects of voters' lives, from education and health care to voting rights.

Democratic gubernatorial nominee Josh Shapiro has been spotlighting the fringe views of state senator Doug Mastriano, his far-right opponent who was involved in Trump's attempts to overturn the 2020 election.

A victory for Trump-backed Mastriano would give the prominent election denier oversight of the state's voting system for the 2024 presidential race.

Like Biden, Trump has visited Pennsylvania twice this year, rallying for Oz and Mastriano most recently in Wilkes-Barre in early September.

The 76-year-old tycoon has already claimed baselessly that the state's elections have been "rigged," echoing his false claims that his own 2020 defeat was the result of widespread fraud.

"As Biden's approval rating plummets, Pennsylvania crime spikes, and Pennsylvanians grapple with a 74 percent hike in heating oil, coupled with record inflation, just weeks away from winter," Trump's office said in a statement.

"The America First Movement offers the Keystone State an alternative vision for America: safe streets, cheap gas, low inflation, and a thriving American economy."

© Agence France-Presse