Kamala Harris will bring her message of joy directly to Donald Trump during a debate that experts say is shaping up to be a critical moment ahead of the November U.S. election.

鈥淭his is perhaps the only time U.S. voters will see Kamala Harris and Donald Trump interact directly on stage together,鈥� said Allison Prasch, an associate professor at the University of Wisconsin.

鈥淚 think this is possibly the most consequential 90 minutes of this entire election season.鈥�

The pair will meet onstage in Philadelphia in the second presidential debate of the campaign. It caps a tumultuous summer for both parties, which saw the attempted assassination of the former president, the complete reconfiguration of the Democratic ticket and struggles among Republicans to respond to their new opponent.

The first presidential debate in June was a pivotal moment for Democrats. Trump easily held his own against a jolting and incoherent President Joe Biden.

The debate accelerated a cascade of concerns over the president鈥檚 mental acuity and ability to win the election, and led Biden to remove himself from the race.

Tuesday鈥檚 faceoff could eclipse June鈥檚 debate as a defining moment of the election.

鈥淚t is quite literally the visual and discursive moment where there two choices are side-by-side,鈥� said Prasch, an expert on U.S. presidential rhetoric.

The stakes are extremely high in the tight race. Harris has been seen polls slightly swing in her favour in some important battleground states as she rides a wave of enthusiasm and excitement since taking the helm of the Democratic ticket. But margins in Pennsylvania, where the debate is happening, remain razor thin.

Matthew Lebo, a specialist in U.S. politics at Western University in London, Ont., said the state is the race鈥檚 biggest battle.

While Philadelphia is generally a Democratic stronghold, Pennsylvania has turned into a battleground state that could be crucial to win the presidency. Trump took it in 2016 before it swung back to Biden in 2020 鈥� both by narrow margins.

Tuesday鈥檚 matchup will put Harris鈥檚 skills to the test with fewer than 60 days until the election. She will have to reach out to voters and layout her campaign鈥檚 policies while holding Trump鈥檚 attacks at bay.

The vice-president is expected to rely on her background as a prosecutor while still leaning into the feelings of hope and joy her campaign has embraced since Biden鈥檚 departure.

Prasch said a successful debate for Harris would also involve clear communication about her policy approaches to the economy, Israel and the war in Gaza, and 鈥渒itchen-table issues鈥� affecting everyday citizens.

Canadians are likely listening for plans around international trade. Whoever wins the election will be in power during the review of the Canada-United States-Mexico trade pact in 2026.

Many expect Harris will follow the path of her predecessor on Canada-U.S. relations. Harris, however, was one of 10 U.S. senators to vote against the trilateral agreement under Trump, saying it didn鈥檛 do enough to protect American workers or the environment.

Trump has floated ideas that would disrupt global trade and has already signalled plans for a 10 per cent tariff on imports.

Lebo said he doesn鈥檛 expect to gain much insight into Canadians鈥� questions Tuesday.

Trump鈥檚 greatest challenge will be staying on message and away from comments that could come across as misogynistic or racist, he said.

In the final moments of the third presidential debate in 2016, Trump memorably called Hillary Clinton 鈥渁 nasty woman.鈥� Facing off against Harris, who has proved a difficult opponent for Trump so far, Lebo said the president鈥檚 comments could veer into similar territory.

鈥淗e goes off script,鈥� Lebo said. 鈥淲hen he鈥檚 on stage with Harris, and she鈥檚 going to have some very good answers, she鈥檚 not going to give the respect he thinks he deserves 鈥� I don鈥檛 know if he will be able to contain himself.鈥�

Ahead of the debate, Trump has criticized both the format and ABC News moderators, claiming he will not be treated fairly. He said on social media that 鈥渘o boxes or artificial lifts will be allowed to stand on鈥� during the debate.

鈥淚t would be a form of cheating, and the Democrats cheat enough,鈥� Trump said Saturday.

Lebo said that rhetoric may work well among Trump鈥檚 base of supporters but it won鈥檛 get him elected.

鈥淗e needs more than his base.鈥�