Outside of the Trump-critical wing of the Republican primary — featuring Christie and Hutchinson, with major appearances by Pence — most of the presidential field has tip-toed around Trump, careful not to attract his ire. But in the last couple weeks, some candidates have started taking on Trump more directly, jeopardizing their prospects as his running mate.

Trump was already unlikely to name a challenger as his VP pick. Of the 19 unique Democratic and Republican presidential nominees since 1972, only four selected a running mate who had run against them in the same year’s primary. And Trump is even more prickly about loyalty than his recent predecessors.

Last week, we analyzed the tweets of the six highest-polling Republican candidates (other than Trump) in 538’s national polling average. Until recently, DeSantis had hardly acknowledged the former president, mentioning Trump just once since Jan. 6, 2021. But that changed last week, when DeSantis published three critical tweets about Trump. Scott, meanwhile, has largely kept his social media tone neutral toward Trump, but we did see a rare direct criticism on the stump last week, when he said Trump’s suggestion that he’d seek a compromise on federal abortion legislation was the wrong approach. Haley’s tone toward Trump shifted earlier this summer, from cautious to directly critical.

Tonight could tell us whether these recent instances of criticism are the beginnings of a new trend in which the GOP field actually tries to topple the front-runner.

—Analysis by Leah Askarinam of 538

Everyone obsesses over horse-race polls, but another important data point in the primary is each candidate’s net favorability rating, or the share of Republicans who view them favorably minus the share who view them unfavorably. In a horse-race poll, respondents can only choose one option, but in reality, there are probably multiple candidates they might consider voting for. Net favorability rating gives us a proxy for how many voters might be open to supporting a given candidate, even if they are not currently a voter’s first choice.

Unsurprisingly, Trump is the most popular candidate in the GOP field, with a net favorability rating of +53 percentage points among Republicans, according to the 538 average. But DeSantis isn’t too far behind him at +43 points, indicating that, although he isn’t many Republicans’ first choice, most of them still like the guy. Ramaswamy, Scott and Haley also have strongly positive net favorability ratings, suggesting that they are the five most plausible Republican nominees. By contrast, Pence and Christie are underwater with Republican voters, making it difficult to see how they can win the nomination.

—Analysis by Nathaniel Rakich of 538

After each debate, we are releasing numbers from new polling with Ipsos and The Washington Post that show who debate viewers thought won the debate. But since this is the second debate of the campaign, we have the benefit of getting a month’s worth of new polls to average since the first debate. Overall, the picture remains largely the same — but there are some minor changes in the race for second place.

538’s latest national polling average has Trump at 54 percent of the vote among likely Republican primary voters nationwide. On the eve of the last debate in late August, Trump was polling at 52 percent. DeSantis is in second at 14 percent; he was at 15 percent heading into the last debate. Following him are: Ramaswamy, who rose to 10 percent following the last debate but has since settled at 6 percent; Haley at 6 percent (up from 3); Pence at 5 percent (up from 4); Christie and Scott at 3 percent each; and everyone else at 1 percent or below.

—Analysis by G. Elliott Morris of 538

538’s Galen Druke traveled to Simi Valley, California — home of the Ronald Reagan Presidential Library and tonight’s debate — to explore why an area that was once a Republican bastion voted for President Biden in 2020 and whether Republicans can win it back.

Foreign policy isn’t quite dominating this debate, but U.S. policy towards China is a prominent theme, and Ramaswamy is bearing down a lot of attacks on that.

Ramaswamy and Haley had a heated moment on the debate stage over the issue of support for Ukraine – which Haley tied to China. When Ramaswamy argued against support for Ukraine, Haley jumped in, saying that a win for Russia is a win for China, and then accused Ramaswamy of liking China.

There was also the memorable TikTok exchange between the two. Haley sniped at Ramaswamy that "every time I hear you, I feel a little bit dumber for what you say” and that he can't be trusted as he justified his choice to join TikTok after being convinced by influencer and boxer Jake Paul to join despite previously calling it "digital fentanyl." TikTok was created by Chinese company ByteDance and lawmakers have scrutinized the app over security concerns. (Ads for TikTok are also airing on Fox Business Network during commercial breaks.)

Scott also accused Ramaswamy of being ‘bought’ by China. He was likely referencing Ramaswamy's company, Roivant Sciences, announcing 5-years ago that it was partnering with China state-owned investment company CITIC Group Corp

And Pence had a similar, pointed attack: “I'm glad Vivek pulled out of his business deal in 2018 in China -- that must have been about the time you decided to start voting in presidential elections."

It’s worth noting that Haley criticized Trump too on this front, in that case for not pursuing China aggressively enough on a host of issues: “This is where President Trump went wrong.”

– ABC News’ Kendall Ross, Libby Cathey, Soorin Kim, Abby Cruz, Nicholas Kerr, and Oren Oppenheim