The Biden-Harris campaign appeared to buy airtime on Fox News ahead of the GOP primary debate, with ad advertisement for the Democrats’ reelection campaign running just minutes before the eight Republican hopefuls took the stage.

The ad, which acted as something of counter-programing to what the GOP hopefuls may discuss during the two-hour debate, touted what the Biden administration sees as its legislative accomplishments, like the American Rescue Plan, the Inflation Reduction Act, the CHIPS and Science Act and the Bipartisan Infrastructure Law.

The spot highlighted the economy under Biden, which is likely contrary to what the Republican primary candidates will raise on stage -- citing Biden's low marks from voters on his handling of economic issues.

–ABC News’ Isabella Murray

I’m monitoring Christie and Scott tonight. Even though the two candidates are polling similarly in the 3 to 4 percent range in FiveThirtyEight’s national polling average, they are a subject in contrasts. Although Pence has been more vocally critical of Trump recently, Christie is the most high-profile, full-on anti-Trump candidate on stage. Realistically, Christie has very little chance at the GOP nomination because he has negative net favorability ratings among Republicans. But that may mean he will focus almost entirely tonight on attacking Trump rather than making appeals to the party base. As a result, Christie could be a popular foil for the contenders on stage who have largely eschewed going after Trump, or have even defended him.

Meanwhile, Scott arguably has one of the highest ceilings of any candidate on stage. He’s not nearly as well-known to GOP voters as Trump or DeSantis, but Scott gets rave reviews from Republicans who are familiar with him. That means tonight is an opportunity for him to expand his reach and also potentially increase the share of GOP primary voters who would consider voting for him. I’ll be interested to see how much he leans into his “happy warrior” image as a means of differentiating himself from DeSantis and Ramaswamy, and whether he tries to rise above the fray to some extent if some of the other candidates — especially Christie — get into verbal fisticuffs.

–Analysis from Geoffrey Skelley

I’ll be watching Ramaswamy tonight, who will be center stage next to DeSantis, a position I did not foresee for the biotech entrepreneur when he first jumped into the race in February. But Ramswamy is on an upswing, soaring from a virtual unknown to garnering nearly 10 percent in FiveThirtyEight’s national polling average. Without Trump on the stage and with DeSantis’s polling average trending downward, is it Ramaswamy’s moment to own it?

Ramaswamy has staked his candidacy on an anti-woke, pro-capitalist message, promising to guide the country at a revolutionary moment by recognizing 10 simple truths. He’s a 38-year-old millennial leaning into the culture wars from the right who often seems to say exactly what pops into his head. This style has so far made him a major candidate. We’ll see tonight whether it appeals to voters. Thirty percent of likely Republican primary voters view him favorably, according to a FiveThirtyEight/Washington Post/Ipsos poll released today, and only 10 percent view him unfavorably. That leaves a huge chunk of voters in the middle who say they don’t know much about him and who may be seeing him speak at length for the first time during the debate. How he performs — whether it involves quoting Eminem or not — could leave a lasting impression. And a newcomer without political experience coming out of nowhere to win the Republican nomination? It’s something we’ve seen before.

–Analysis from Monica Potts

I’m tracking Burgum and Haley tonight. In another universe, the two might be top-tier contenders — a billionaire two-term governor from the heartland with great hair, and a trailblazing former governor and U.N. Ambassador long seen as a rising star in the GOP — but neither have made much of an impact on the race so far. Haley had a brief moment of prominence in February, when she was the first non-Trump candidate to jump in the race, but has since polled consistently in the low single digits. Haley will be the only woman on stage, and she may try to play to her strengths by drawing sharp foreign policy contrasts with higher-polling contenders like DeSantis and Ramaswamy (who she went after over Israel policy this week). Most of the attention Burgum has received surrounds his innovative tactic of paying people to donate to his campaign. The North Dakotan has been polling marginally better in Iowa and New Hampshire, where he’s spent millions on TV ads. If he makes it to this debate — he was in the emergency room earlier due to a basketball-related injury — his task will be to tell voters who he is, since he’s one of the least-known people on the stage, and maybe sell his Trump-free economic argument.

–Analysis from Jacob Rubashkin, Inside Elections

The 2024 presidential election has been underway for months, but with tonight’s first Republican primary debate in Milwaukee, we’re kicking off a new phase of the campaign. In these final five months before voting begins, we’re going to have monthly debates, dashes through Iowa, big-money fundraisers — and probably a culling of the field.

Tonight’s debate will feature eight candidates. Front-runner Donald Trump, who has a 37-percentage-point lead in FiveThirtyEight’s national polling average, will not be among them, as he has opted to skip the debate. Instead, we will hear from Florida Gov. Ron DeSantis; entrepreneur Vivek Ramaswamy; former Vice President Mike Pence; Sen. Tim Scott; former U.N. Ambassador Nikki Haley; former Arkansas Gov. Asa Hutchinson; North Dakota Gov. Doug Burgum; and former New Jersey Gov. Chris Christie.

There are a few threads and tensions we’re going to be watching tonight. First, in the absence of Trump, will his challengers’ attempts to position themselves as a more favorable option fall flat? How will candidates make their case for being a better alternative to Trump without having the contrast on stage beside them? Secondly, what sort of friction will we see between the anti-Trump candidates and those running on more similar ideas or rhetoric?

We’ll keep an eye on all this and more. You’ll be hearing from the crew at FiveThirtyEight and ABC News — as well as some new friends at PolitiFact — throughout the night. The Fox News-hosted debate begins at 9 p.m. Eastern, and we have some colleagues on the ground in Milwaukee who will ensure you’re going in with everything you need to know.

Thanks for joining us. Stay tuned as we update the blog with real-time analysis, charts, thoughts, questions, ideas, idle fancies and more. Leading up until the debate starts, we’ll run through the state of the race so far. Once the debate starts, we’ll make sure we’re addressing everything happening on stage, including what issues the candidates talk about, who they attack, how they position themselves and what Americans think.

-Analysis by Maya Sweedler of FiveThirtyEight