Democratic President Joe Biden and his Republican predecessor, Donald Trump, square off on Thursday at an unusually early debate that will offer voters a side-by-side look at the two oldest candidates ever to seek the country’s highest office.

The 90-minute televised debate, the first between a sitting president and a former one, will air at 9 p.m. ET (0100 GMT on Friday) on CNN and is expected to draw a huge audience. A record 84 million watched Trump’s first debate in 2016 against Hillary Clinton.

Both men enter the debate in Atlanta with political vulnerabilities that present a mix of risk and opportunity. The debate takes place far earlier than normal - more than four months before the Nov. 5 Election Day - and against a backdrop of national opinion polls showing the two men in a dead heat.

The clash also arrives at a moment of profound polarization and deep-seated anxiety among voters about the state of American politics. Two-thirds of voters said in a May Reuters/Ipsos poll that they were concerned violence could follow the election, nearly four years after a mob of Trump supporters stormed the U.S. Capitol.

Trump, 78, will take the stage as a felon who still faces a trio of criminal cases, including charges related to his efforts to overturn the 2020 election. The former president, who has suggested he will punish his political enemies if elected, will need to demonstrate to undecided voters that he does not pose a mortal threat to democracy, as Biden asserts.

Biden, 81, is under intense pressure to avoid verbal stumbles and deliver a forceful debate performance, after months of Republican assertions his faculties have dulled with age.

Ahead of the debate, each campaign was to launch a new ad campaign attacking its rival. The Biden campaign said it would launch a new ad blitz in the Atlanta area focused on what a second Trump presidency could look like, including threats to reproductive freedom, a ban on abortion, cuts to Social Security and undermining democracy.

The Trump campaign said it would release two new television ads during the debate, one targeting the economy, illegal immigration and crime and the other focused on Biden’s physical stumbles and saying he “won’t make it four more years in the White House.”

High stakes

When they take the stage, either Biden nor Trump will have much room for error, with the opposing campaign likely to seize on any slip-up as evidence of cognitive decline.

Aaron Kall, a University of Michigan professor and an expert on presidential debates, said this “might be the most highly anticipated and important” one ever, given the closeness of the race, the country’s deep political polarization and the potential for a lasting gaffe.

For Biden in particular, he said, a moment of confusion or forgetfulness would prompt “endless news cycles” about his age and refuel speculation about a possible replacement ahead of the Democratic National Convention in August.

While national polls show a tied race, Biden has trailed Trump in polls of most battleground states and recently saw his financial edge erased after Trump was convicted in connection with hush money payments made to a porn star.

“Biden needs a change in the status quo, and this debate is his best opportunity yet to do it,” said Jacob Rubashkin, an elections analyst at the nonpartisan website Inside Elections.

“Right now, voters are looking at this race more like a referendum on Biden than a choice election, and that’s dangerous territory for him to be in. But in the debate he can drive home the contrast angle - and Trump will be in the spotlight as well.”

Neither Biden nor Trump is popular, and many Americans remain deeply ambivalent about their choices. About a fifth of voters say they have not picked a candidate, are leaning toward a third-party candidate or may sit the election out, the latest Reuters/Ipsos poll showed.

“They’re horrible candidates,” said Kathy Elder, a 59-year-old sales manager who voted for Trump in 2016 before switching to Biden in 2020.

Elder will be watching Thursday’s debate to try to decide which way to go this year, but she said she cringes whenever they speak – for different reasons.

When it comes to Biden, she said, “Can he speed this up and actually talk?” As for Trump, she said, “What the hell is going to come out of his mouth?”

Biden and Trump have made little effort to disguise their mutual dislike. During their first debate in 2020, Trump aggressively talked over Biden in a performance that turned off many voters.

CNN will attempt to avoid cross-talk by muting the candidates’ microphones when it is not their turn to speak. The debate will also take place without an audience, and neither candidate is allowed to bring prepared notes or props, though they will have a pen and paper.

Biden advisers say he will emphasize Trump’s role in threatening abortion access, portray him as a danger to democratic norms and remind voters of Trump’s often chaotic 2017-2021 term in office.

Biden campaign spokesperson, Michael Tyler, said the debate would show the contrast between Biden’s efforts to help Americans and Trump’s “unhinged campaign of revenge and retribution.”

Trump will focus on Biden’s stewardship of the southern U.S. border in the face of record numbers of migrants crossing illegally as well as the economy, particularly inflation, while also questioning his world leadership at a time of war in Gaza and Ukraine, Trump advisers said.

“Thursday night is going to be a great opportunity for President Trump to highlight his strength with Joe Biden’s weakness, highlight his record of success with Joe Biden’s record of failures,” campaign spokesperson Karoline Leavitt said.

The second and final debate in this year’s campaign is scheduled for September.