President Trump and Democratic nominee Joe Biden traded insults on Saturday as the two spent the day in key battleground states on the campaign trail.

The president spent the day campaigning Pennsylvania holding four separate rallies in the state, while Democratic nominee Joe Biden hit the trail for the first time with former President Barack Obama in Michigan just three days before Election Day. 

Trump's focus on Pennsylvania underscores both his campaign's belief that the race there is close and the reality that he may be hard pressed to win re-election without carrying the Keystone State.

Biden and Obama traveled to Michigan, seeking to shore up support in a state that Democrats neglected and narrowly lost four years ago. The two held drive-in rallies in Flint and Detroit.

The tone and substance of the events reflected the bitter nature of the campaign as Election Day nears.

Trump and Biden have for months made clear their personal dislike for one another, and that was on display at competing events on Saturday.

The president in Newton, Pa., implied Biden had plastic surgery on his face, telling the crowd "he should get a little bit of a larger frame," referring to the aviator sunglasses Biden wears.

"He doesn't want to have people look at that surgery on the eyes," Trump added.

At a subsequent rally in Reading, Pa., Trump invoked Biden's words in 2018 that he would beat the president up if the two were in high school, citing Trump's comments about sexually assaulting women on an "Access Hollywood" recording from 2005.

"Do you remember like a year ago, year and a half ago, he said, 'I’d like to take him to the back of the barn.' A slight slap," Trump said, miming hitting someone with a backhand and suggesting he could easily knock Biden over.

Biden, speaking at nearly the same time, happened to harken back to his high school comments.

"When you were in high school wouldn't you have liked to take a shot?" Biden asked supporters in Flint, Mich.

The former vice president derided Trump as "Vladimir Putin's puppy" and a president who "commands virtually no respect on the international stage."

"Everybody knows who Donald Trump is. Let’s keep showing them who we are," Biden said. "We choose hope over fear. Unity over division. Science over fiction. And yes, truth over lies."

But it was Obama who has savaged Trump most thoroughly in his appearances on the campaign trail, and that remained true on Saturday as he appeared alongside Biden for the first time during this campaign.

Obama has let loose on Trump in a way that is unusual for a former president to talk about his successor, reflecting both the personal animosity and the way in which Trump has aimed to undo most of Obama's accomplishments.

The 44th president bashed Trump as incompetent and unwilling to adapt to the requirements of the job. He excoriated the administration over its handling of the ongoing pandemic, which has killed more than 230,000 Americans.

And Obama mocked Trump for his infatuation with crowd size, something the president likes to point out about his rallies to argue voter enthusiasm is on his side.

“What is his obsession by the way with crowd size? You notice that?” Obama said. This is the one measure he has of success. He’s still worried about his inauguration crowd being smaller than mine. It really bugged him. He’s still talking about that.”

“Did nobody come to his birthday party when he was a kid? Is he traumatized?” the former president continued. “What’s with crowds?”

Obama, who remains extremely popular within the Democratic Party, has emerged as an active surrogate for his former vice president. He appeared in Florida and Pennsylvania over the past week, and he is expected to campaign in Florida and Georgia on Monday.

Beyond the attack lines, the visits were intended to solidify support in two states that are critical for each respective campaign.

Trump won Pennsylvania in 2016 by just 44,000 votes, and his path to re-election becomes increasingly narrow if he's unable to win the Keystone State again. Polls show Biden with a slight lead in the state.

In a transparent attempt to score points in Pennsylvania, Trump on Saturday announced his administration would conduct a review of the economic benefits of hydraulic fracking and potential economic consequences should the practice be halted.

Biden has said he would not ban fracking, but Trump and his advisers have argued that he would buckle to pressure to do so once in office.

Biden, meanwhile, has maintained a steady lead in Michigan, according to polls. But many Democrats remain nervous about being overconfident and are aware of 2016, when Hillary Clinton struggled to turn out voters in the state, which she lost to Trump by less than 11,000 votes.

Trump, in a nod to Michigan's importance, will hold three rallies there in the next two days. And Biden, who is hoping to shut the door on Trump in Pennsylvania, will campaign in the Keystone State on Sunday and Monday.