By Jeremy Diamond, CNN

(CNN) -- President Donald Trump is set to rally his supporters Monday in central Pennsylvania, a key battleground state that has been flashing warning signs for Trump's reelection campaign.

The President's challenges in Rust Belt states, critical to his narrow 2016 victory, are at the forefront of advisers' minds. Polling conducted by the Trump campaign last month confirmed cracks in his support in Pennsylvania and Michigan, reinforcing warnings from the 2018 midterms, when Democrats took back the House.

Trump hopes rallies will be part of the solution to shoring up his support. Frustrated by the increasing amount of TV news coverage the Democratic presidential candidates are attracting, Trump is looking to shift the spotlight back to his campaign.

The Trump campaign is now eyeing a cluster of rallies next month to formally kick off Trump's reelection campaign, a source close to the campaign told CNN.

The President will arrive in Pennsylvania just two days after former Vice President Joe Biden rallied supporters in Philadelphia, casting his bid for the Democratic nomination in terms of his ability to defeat Trump in a general election.

And Biden is not the only candidate threatening Trump in the crucial state, with Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts also leading Trump in hypothetical head-to-head matchups, according to a recent Quinnipiac University poll.

Trump has focused his energy on Biden -- who leads Trump in Pennsylvania by 11 points in the Quinnipiac survey -- more than any other candidate. The President has frequently knocked Biden on Twitter -- including on Monday -- in an effort to cut down his would-be opponent. That's despite the fact that some of his own advisers worry Trump's fire will only serve to bolster Biden's standing in the crowded Democratic field.

Beyond Pennsylvania, Trump is looking to kick his campaign into a higher gear next month ahead of the first Democratic debates.

Trump campaign aides are also finalizing plans for a blitz of rallies beginning around June 16, the anniversary of Trump's 2016 campaign launch, the source said. Trump's reelection campaign has been in existence since his first day in office and he has been holding campaign rallies since early in his term, but campaign aides note that Trump has not yet held a formal kickoff rally. Axios first reported on the plans for the campaign launch.

Aides expect Trump to grow more restless as he watches Democrats take over his television. He has already begun publicly airing some of those grievances, tweeting on Sunday to complain about Fox News' town hall with Pete Buttigieg, the South Bend, Indiana, mayor and 2020 Democratic presidential candidate.

The campaign is also debating ways to counterprogram the Democratic debates that will start soon, eyeing the possibility of ramping up Trump's rallies, including potentially hosting some in the same city as the debates.

Trump's rally on Monday also comes on the eve of a special election for Pennsylvania's 12th Congressional District, a conservative stronghold where Trump will be looking to ensure Republican Fred Keller keeps the seat in Republican hands.