5 takeaways from striking Biden-Trump presidential debate

President Joe Biden delivered a halting performance in Thursday's debate with former President Donald Trump -- raising new questions about his future in an event largely viewed as a test of the two candidates' fitness for office.

The two debated a slate of policies on stage in Atlanta, though little new ground was broken. Much of the focus was instead on how the 81-year-old president and his 78-year-old predecessor would handle another four years in the Oval Office -- with Democrats left worrying about Biden's performance.

Here are five takeaways from Thursday night's clash.

Biden has slow start

Biden came on stage with a raspy voice and repeatedly stumbled through some of his answers early on.

When asked a question on the national debt, he said there are "thousands of trillionaires" before correcting himself, for instance.

But perhaps his biggest stumble came about 12 minutes into the debate, when he paused for six seconds after discussing ways the country could have spent money provided by taxes on wealthy Americans that would have been accrued were it not for tax cuts passed by Trump during his term.

"We'd be able to help make sure that all those things we need to do, child care, elder care, making sure that we continue to strengthen our health care system. Making sure that we’re able to make every single solitary person eligible for what I’ve been able to do with the -- with the COVID, excuse me -- dealing with everything we have to do with, look … we finally beat Medicare," Biden said, appearing to lose his train of thought at the end of his answer.

The moment precisely highlighted a major theme heading into the debate -- Biden's mental acuity and fitness for office -- and sent Republicans celebrating.

"Game over!!!" Bryan Lanza, a former Trump campaign aide who remains in touch with the former president's current team, texted ABC News.

Democrats, too, sounded a negative note.

"A few words the Biden team should look up: espresso; and honey and lemon for the throat," said one high-level Democratic strategist.

Biden appeared to pick up steam as the debate dragged on, dubbing Trump a "convicted felon" and mixing it up over their records, but the president's performance was less energetic than that of his opponent.

Biden team spins his performance

Biden's team quickly worked to spin is performance, seemingly recognizing that he likely did not present himself as Democrats had hoped.

The campaign said the president had a cold, but tested negative for COVID-19, though it's unclear why that news wasn't announced beforehand. The campaign told ABC News' Mary Bruce that they're feeling fine but conceded he had a slow start.

"Tonight, President Biden presented a positive and winning vision for the future of America -- one in which every American has a fair shot at the American dream, where every one of our rights are protected, and where our president fights to strengthen our democracy -- not to tear it down," said Biden campaign chair Jen O'Malley Dillon.

Biden's allies echoed past support for the president, allowing that the night likely did not go as planned but that the president will remain the nominee.

"I don't care, I'm going to stick with Joe Biden," former Democratic National Committee chair Donna Brazile said on ABC News Live.

Trump (mostly) keeps his cool

Operatives were eyeing which version of Trump would appear on stage Thursday: the brawler who repeatedly interrupted Biden at their first 2020 debate? Or would he be a staid presence that would be interpreted as more presidential?

Largely, the latter ended up as a more apt description of Trump's performance.

To be certain, Trump veered into some of his more bombastic rhetoric -- exaggerating the state of the economy under his term and the number of border crossings and crimes taking place under Biden; saying Biden "could be a convicted felon"; and accusing the president of "going after his political opponent because he can't win fair and square."

But he did not center his arguments around unfounded claims of election fraud or repeated attacks on Hunter Biden, the president's only surviving son who was recently convicted on felony gun charges. Oftentimes, he returned his answers to favorable topics for him like inflation and immigration, including on a question regarding the Jan. 6 riot on Capitol Hill.

"Let me tell you about Jan. 6. On Jan. 6, we had a great border. Nobody coming through. Very few on Jan. 6. We were energy independent on Jan. 6, we had the lowest taxes ever. We had the lowest regulations ever," he said.

Trump's campaign swiftly declared victory.

"Tonight President Trump delivered the greatest debate performance and victory in history to the largest voter audience in history, making clear exactly how he will improve the lives of every American," top Trump campaign hands Chris LaCivita and Susie Wiles said in a statement.

"Joe Biden on the other hand showed exactly why he deserves to be fired," they said. "Despite taking a week-long vacation at Camp David to prepare for the debate, Biden was unable to defend his disastrous record on the economy and the border."

New rules impose cleaner debate, but less pushback

Thursday's debate had novel rules that were largely successful in imposing a cleaner debate than in the past.

No audience was in the studio to interrupt with applause or boos, and microphones were cut off when candidates were not recognized to speak.

The result was a debate with little crosstalk or disruptions, a stark departure from primary debates earlier this year and debates during 2020, when crosstalk made the candidates' comments essentially illegible.

However, while the more rigid format helped move the conversation along, there was minimal pushback from moderators Jake Tapper and Dana Bash, who at times opted to move forward rather than correct falsehoods or push for a direct answer to their initial question if a candidate had already used up their time for a response.

And the conversation still veered into the outlandish at times, with both candidates sparring for a few minutes toward the end over their golfing abilities.

"Let's not act like children," Trump said.

Lots of policy talk, but little new ground

The structure also helped the moderators and candidates stick to policy, but little new ground was broken on the contenders' stances on main issues.

Biden vowed to reinstate Roe v. Wade and protect abortion rights, raise taxes on wealthy Americans and support Ukraine.

Trump defended his tax cuts, said he would force European allies to boost their own efforts to back Ukraine in its fight against Russia and declared that Israel should be able to continue its military operations in Gaza.

However, voters will be left with little new information about what the two candidates believe on key issues that they haven't heard on the campaign trail.