An alternate Republican elector must now cast votes for Trump and Vice President Mike Pence in Florida after state Sen. Wilton Simpson, the newly elected Senate president, tested positive for coronavirus and is no longer able to attend the ceremony.

Katie Betta, a spokesperson for Simpson, confirmed that the senator took a test Sunday night that came back positive. Better said Simpson tests regularly because he travels often, both for his work with the Senate and his personal business. She said that he is experiencing "some very mild symptoms," which she said the senator “compared ... to the symptoms he's had before with allergies or like a very mild head cold."

ABC News has asked the Republican Party of Florida who the alternate will be ahead of the electors meeting at 2 p.m.

-ABC News' Kendall Karson

Among the the 538 electors who formally cast their votes for president and vice president Monday, there are some notable faces.

In New York, former President Bill Clinton and former Secretary of State Hillary Clinton are among the electors, along with New York Democratic Gov. Andrew Cuomo.

In Georgia, former Democratic gubernatorial candidate Stacey Abrams is an elector, as is Nikema Williams, the Democratic congresswoman-elect for the district previously represented by late civil rights icon and longtime Rep. John Lewis.

South Dakota GOP Gov. Kristi Noem is an elector in her state, and Chicago's Democratic Mayor Lori Lightfoot already cast her for Biden as an elector for Illinois.

The electors, who reflect the distribution of power across the states based on congressional representation, are decided every four years by each state's political parties in the months before the presidential election. The process for choosing the electors varies by state, with some nominating their electors at party conventions, while others leave it to voters to elect them during the primary process.

-ABC News' Kendall Karson

The Electoral College kicked off voting in four states at 10 a.m. with Indiana, New Hampshire, Tennessee and Vermont being the first states slated to cast electors. While the Electoral College meets on the same day, each state determines when and where its electors meet.

Biden is expected to reach 270 votes -- the number need to win the presidency -- when California's electors vote at 5 p.m.

When will electors vote in the key battlegrounds?

Starting at 10 a.m. ET and rolling through the day, electors cast their votes on Monday for president and vice president in state capitols across the country -- another step towards enshrining Biden's victory and a step that will happen irrespective of Trump's last-ditch effort to overturn the result.

Biden is slated to deliver remarks on the Electoral College vote certification and "the strength and resilience of our democracy" from Wilmington, Delaware, around 8 p.m. Trump has an executive order signing on his schedule and has continued to air his grievances with the election on Twitter.

Monday's casting of electoral votes is traditionally little more than a formality with federal law requiring electors to meet the "Monday after the second Wednesday in December of presidential election years" -- but as Trump resists his defeat at every turn and wages long-shot legal battles with baseless claims of fraud, the meeting comes at a tense and fragile moment for the country's democratic institutions. Biden’s margin of victory in the Electoral College is expected to be 306 votes to Trump's 232, if there are no surprises.

Electors gather in each of their respective states and the nation's capital to cast separate paper ballots for president and vice president at places determined by the state legislature. Most of the meetings will be on cam via livestream and in the middle of a pandemic, but in Michigan, another threat interrupted the plan.

Officials announced late Sunday that the state legislature's office buildings will be closed due to "credible threats of violence" after Michigan’s 16 electors meeting were scheduled to cast their ballots at the state Capitol in Lansing. In Arizona, electors were given an undisclosed location to meet and cast their ballots in an effort to avoid any confrontation with protesters.

It's unclear how Trump's allies and supporters, particularly those in Congress who have fallen in line with his refusal to accept the loss -- despite Friday's final rejection at the Supreme Court -- will respond to Biden's victory once the Electoral College voting is complete.

-ABC News' Kendall Karson

Wrapping up Saturday's event announcing members of the incoming administration's climate and energy team, Vice President-elect Kamala Harris emphasized how forceful Biden's plan is in fighting climate change.

"Part of the reason I was so proud to join Joe Biden as his running mate was because he was proposing one of the most ambitious climate plans in history, a plan to secure carbon pollution-free electricity by 2035, a plan to achieve net-zero emissions no later than 2050, a plan to invest in a clean energy future and create millions of good-paying union jobs along the way," she said.

"And the team that President-elect Biden and I are announcing today will help make that plan a reality," Harris added.

The vice president-elect said the appointees and nominees announced Saturday represented some of the country's "most seasoned public servants and climate experts" and reflected the "very best of America."

"They are compassionate leaders who understand that, ultimately, addressing climate change is about building safer communities and healthier communities and thriving communities for all Americans," Harris said.