Biden faces high election-year stakes in State of the Union address: ANALYSIS

President Joe Biden will face one of his biggest national audiences of the year Thursday night when he delivers his third State of the Union address.

While the speech is a constitutionally-mandated tradition of the presidency, and not supposed to be a campaign speech, with the 2024 election now in full swing, it's a key opportunity for Biden to make his case for a second term to millions of Americans.

His record and vision for the country -- touting the economy and the need to pass aid to Ukraine -- will form the substance of his speech, but equally as important will be his performance as he attempts to quell doubts about whether he can handle another four years.

"He will need to show that he is vigorous, in command, and a happy warrior," Michael Waldman, chief speechwriter for former President Bill Clinton, told ABC News. "People will be paying more attention than usual to his presentation given the criticism of his age."

Biden, 81, is viewed as too old to serve by an overwhelming majority of Americans, according to a recent poll from ABC News and Ipsos. While Donald Trump at 77 years old faces some scrutiny over his age (59% of Americans think both Biden and Trump are too old) it is a greater weakness for Biden.

Biden's response to voter concerns has ranged from self-deprecating jokes about how long he's been around ("I'm 198 years old") to criticizing Trump's ideas as what's truly outdated.

He is expected to further contrast his agenda with that of the Republican Party -- and by implication Trump's -- on Thursday night. A White House official told ABC News he will discuss his economic vision, reproductive rights, the state of democracy and more.

The address comes just two days after Super Tuesday solidified Trump's status as the presumptive GOP nominee. Republican Nikki Haley exited the race, as did Democrat Dean Phillips, essentially clearing the way for a Biden-Trump rematch.

"This is the starting gun for the general election," said Jim Kessler, the vice president of policy for the center-left think tank Third Way. "In an election year, the State of the Union is often the place where the incumbent begins to make the case for deserving reelection. It's a high-stakes speech ... the race is not going to be determined by what the president says but these are opening steps and they matter."

A White House official was tight-lipped about whether Biden would call out Trump by name.

Biden spent the weekend at Camp David with historian Jon Meacham preparing for the prime-time address, which for the first time will be streamed on Instagram to the president's nearly 19 million followers.

"One of the big questions that presidents face in any State of the Union but especially in an election year, is how much swagger they should have, how braggadocious they should be," said Waldman, who now leads the Brennan Center for Justice. Waldman encouraged Biden to lean into the positive results from his administration, saying presidents who are "confident and have a story to tell" fare better in their reelection bids.

Last year, Biden's address -- delivered amid speculation about whether he would run for reelection -- was well-received in large part because of a feisty, ad-libbed exchange with raucous Republican lawmakers, who had just taken control of the House.

When Biden criticized a GOP senator's proposal to change Social Security and Medicare, it prompted jeers from Republicans. Georgia Rep. Marjorie Taylor Greene even shouted out "liar."

He quickly turned the tables, going off script to trap Republicans into committing to leaving the popular entitlement programs as is.

"So, folks, as we all apparently agree, Social Security and Medicare is off the books now, right?" he shot back to applause, in what became his most memorable moment. "They're not to be touched? All right. All right. We got unanimity!"

Now, he faces an even more hostile GOP largely aligned with Trump.

Notably, though, this time around, House Speaker Mike Johnson has urged his caucus to show restraint. During a House Republican Conference meeting on Wednesday, Johnson told members "decorum is the order of the day" and discouraged them from repeating what happened, sources told ABC News.

"You saw the president last year when some Republican members behaved in a way that was, I would say disrespectful, and he handled that," White House press secretary Karine Jean-Pierre said Wednesday. "And so, the president's ready for anything. He's ready for anything."

Kessler said he expected Biden to do well again, calling the venue "home turf" for Biden, who served as a senator for nearly four decades and as vice president for eight years.

However, he said questions about Biden's age won't be answered in a single night.

"When it's when an issue is top of mind for voters, it's something that he'll continually need to address," he said, adding he is in the "Let Joe Be Joe" camp, which means getting him out in public in less formal settings and having him lean on his centrist roots.

"Those events need to showcase his personality, energy and vibrancy, but also his natural ideology," Kessler said.

Despite all the speculation about Biden's big speech, as of Wednesday night, the White House had released few specifics.

When reporters earlier in the week asked Biden, "What are you preparing for your State of the Union speech?" he replied, simply, "You'll hear me on Thursday."

ABC News' Lauren Peller and Selina Wang contributed to this report.