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.

Several of the Republicans on stage touted their prioritization of border security should they win the presidency.

“What I’ll do on day one is sign an executive order to send the National Guard to partner with Customs and Border Patrol to make sure that we stop the flow of fentanyl over the border,” Christie said first. He expanded his answer to include his plans to fill the “6 million vacant jobs” in the country with legal immigrants.

“This border is going to be a day one issue for me as president,” DeSantis said, also noting that he’d administer the remain in Mexico policy, which requires migrants seeking asylum to remain in Mexico until their US immigration court date.

“We're going to declare a national emergency. Yes, we'll build the wall. We’ll do remain in Mexico,” DeSantis said.

Pence spoke about his role in negotiating the remain in Mexico policy of the Trump administration.

“I negotiated the remain in Mexico policy with the Mexican government. We used economic power to bring the Mexican government to the table. We build hundreds of miles of border wall and despite what's said here today, we reduced illegal immigration and asylum abuse by 90%. And as President of the United States, I can do it again,” he said. “I'm going to be ready on day one to get Congress to step up, secure the southern border of the United States.”

Haley cosigned on the Remain in Mexico Policy.

“Let's go back to Remain in Mexico policy,” she said. “Instead of catch and release. Let's go to catch and deport.”
Ramaswamy said that he was supportive of most of his opponents’ border positions, but that he’d go a step further: he’d end birthright citizenship for the kids of illegal immigrants.

“The Republicans on the stage are on the right side of this issue, militarize the southern border, stop funding sanctuary cities and end foreign aid to Mexico and Central America to end the incentives to come across,” Ramaswamy said.

--ABC News' Isabella Murray