Election 2020 updates: Obama, Biden finish day of campaigning in Detroit

President Trump's Pennsylvania rally blitz on Saturday is primarily taking him to GOP strongholds to excite supporters and activate Republicans who may not have voted four years ago -- a bid to supercharge Election Day turnout to offset any Democratic gains in the suburbs.

Stop 1: Bucks County 

Bucks County is 25 miles north of Philadelphia and the fourth-most-populous county in the state. It's Trump's only stop on Saturday to an area that isn't reliably Republican. While it's gone blue in every presidential election since 2000, the margin has tightened in recent years. Hillary Clinton won Bucks County by fewer than 3,000 votes in 2016. Demographically, Bucks County is 88.1% white, with 40.5% of residents holding bachelor's degrees or higher, according to the 2019 census. 

Stop 2: Berks County
Trump stopped next in Reading, the seat of Berks County and the fifth-largest city in Pennsylvania. Berks County came close to flipping for former President Barack Obama in 2012, where he lost by under 2,000 votes. Trump closed that gap four years ago, seizing the lion's share of the vote by 18,000. Demographically, Berks County is a little more diverse than Bucks. The census estimates that as of 2019, 86.9% of Berks County residents identified as white and 22.5% identified as Latino -- compared to 5.7% in Bucks County. Only about 24.5% of Berks County residents hold degrees.

Stop 3: Butler County 
Trump will travel next to Butler County in western Pennsylvania, another Republican stronghold since 2000. He handily trounced Clinton here in 2016, receiving more than 61,000 votes compared to Clinton's 26,834. According to The Associated Press, more than 10,000 Republicans in the county did not vote in 2016. Butler County is overwhelmingly white, according to census estimates (95.8%), and only 35.4% of residents hold bachelor's degrees or higher.     

Stop 4: Lycoming County
The final stop will be Montoursville in Lycoming County -- yet another GOP area that has not been hotly contested to as far back as 2000. 

Bonus: Luzerne County
First lady Melania Trump was also heading to Wapwallopen in Luzerne County this evening -- one of three "pivot" counties that went for Trump four years ago after backing Obama twice. 

-ABC News' Benjamin Siegel

Former Vice President Joe Biden tested negative for COVID-19 on Saturday, according to the pool.

This was his 18th negative test announced since Trump's positive diagnosis earlier this month.

-ABC News' John Verhovek

Former President Barack Obama and Vice President Joe Biden are campaigning in Detroit and Flint, Michigan, Saturday in their first in-person appearance together on the 2020 campaign trail. 

At the first event in Flint, Obama removed his “VOTE” mask to tell the drive-in crowd, “this Tuesday, everything is on the line.”

“Our jobs are on the line, our health care is on the line,” Obama said.

Obama bashed President Trump's response to the pandemic, saying, "If Trump were focused on COVID from the beginning, cases wouldn't be reaching new record highs."

"Trump cares about feeding his ego," the former president said.

"Joe's not going to call scientists idiots," Obama said. "He's not going to host super-spreader events."

"Tweeting at the TV doesn't fix things," Obama said, but "Biden has concrete plans."

"Joe's plan will guarantee paid sick leave for workers and parents affected by the pandemic. He'll make sure the small businesses in every community ... can reopen safely," he said.

Biden then joined Obama, telling the crowd, "It's time for Donald Trump to pack his bags and go home. We're done with the chaos ... the failure, the refusal to take any responsibility."

"Imagine where we'd be if we had a president who wore a mask instead of mocking it," Biden said.

Both Detroit and Flint are Democratic strongholds. But in 2016, declines among some of the Democrats’ core constituencies helped Trump win Michigan by under 11,000 votes. 

Hillary Clinton won both Wayne and Genesee counties, home to Detroit and Flint, but her margins were significantly less than Obama in 2012. 

-ABC News' Kendall Karson

First Lady Melania Trump made a campaign visit to Wisconsin Saturday, stressing that the pandemic “is not a partisan issue” and claiming “the Democrats want to project feelings of fear and doubt, purely for political reasons."

As the U.S. reaches its highest number of daily coronavirus cases, the first lady said, “We have made great progress in our fight against COVID-19… When COVID-19 invaded our country, we first had to learn what it was, how it spreads and how to prevent it.”

“Joe Biden said this will be a dark winter … his solution is to move backwards and to shut things down,” she said. “When my husband talks about the future, it is filled with continued possibility and forward thinking.”