Tonight’s debate is from 9 to 11 p.m. Here’s how to tune in.

The first debate last month in Milwaukee was a breakout moment for Vivek Ramaswamy, a wealthy entrepreneur and a political newcomer, but it also elevated Nikki Haley, the former South Carolina governor and ambassador to the United Nations. What it didn’t do is diminish Mr. Trump’s lead.

Here’s what to watch for in the second debate.

Can DeSantis reset (again)?

For months, Gov. Ron DeSantis of Florida was widely seen as the strongest challenger to Mr. Trump. But after a first debate in which Mr. DeSantis was largely relegated to the sidelines, his standing in the race has sunk. Recent surveys in Iowa and New Hampshire show that Mr. DeSantis has lost as much as half of his support, falling to third place — or lower. Some of his biggest longtime donors have of late grown reluctant to put more money into a campaign that seems to be headed in the wrong direction.

To rebuild his momentum, Mr. DeSantis will need to do more on the debate stage than simply avoid a major misstep. Some strong exchanges, particularly with Mr. Ramaswamy, who is competing for some of the same hard-right voters, could help Mr. DeSantis stem his losses.

The Trump factor

Mr. Trump, who is under four criminal indictments, skipped the first debate and emerged much as he had entered: as the overwhelmingly dominant figure in the primary race. His opponents mostly jostled for position among themselves, declining to take significant swings at the front-runner in absentia. In the post-debate polling, Mr. Trump gained more support than any of the candidates who had appeared on the stage.

Since then, as his legal cases play out in the courts, Mr. Trump has grown more extreme, and violent, in his rhetoric. He has suggested that Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, should be executed for treason, accused “liberal Jews” of voting to “destroy” America and Israel and seemed to threaten the judges and prosecutors in the felony cases against him.

So far, his rivals have not used those attacks to go after the front-runner as extreme, but with the first ballots to be cast in Iowa in January, time is running out. The Wednesday debate could be among the lower-polling candidates’ last chances to take aim before a large audience, as the Republican National Committee’s criteria to make the next debate stage are expected to become even more strict. It remains to be seen whether the second debate will persuade top donors still on the sidelines to consolidate behind an alternative to Mr. Trump.

Rather than attending the debate, Mr. Trump will appear with autoworkers in Detroit.

How Scott and Haley perform

Mr. Ramaswamy might have grabbed headlines with a pugnacious performance the last time around, but Ms. Haley had arguably the best night.

She distinguished herself with her answers on abortion and foreign policy while seizing the opportunity to position herself as the “adult in the room” as her male rivals bickered. She raised more than $1 million over the 72 hours that followed the event, winning over Republican donors who had been looking for a plausible alternative to Mr. Trump. And she elevated herself over Senator Tim Scott, a fellow South Carolinian, as the next-generation conservative who could potentially appeal to independents and some disaffected Democrats.

Mr. Scott faded on the stage in Milwaukee. But while it’s critical for him to make a splash at the Reagan Library in order to eat into Ms. Haley’s gains, any spotlight-grabbing moments cannot tarnish his persona as the “happy warrior” with the winning smile and the hopeful message. A bad night, or just an invisible night, for Mr. Scott would dim hopes of a resurgence.

Can the more vocal Trump critics make a case?

Former Vice President Mike Pence and Chris Christie, the former governor of New Jersey, have tried to position themselves as the “anti-Trumps.” Mr. Christie is the loudest castigator of the former president as a threat to the nation, while Mr. Pence has denounced his former running mate as a false conservative, soft on abortion and too populist on trade and foreign policy. Neither argument has gained traction with voters so far.

For both men, the debate will be a chance to find an anti-Trump message that actually appeals to Republican voters. Mr. Christie tried to use his trademark slashing style in Milwaukee, only to be booed by an audience that registered its loyalty to Mr. Trump. The audience Wednesday night could prove to be more sympathetic, or at least more polite, allowing more of the former governor’s blows to land.

Shutdown politics

The federal government appears to be barreling toward a shutdown this Sunday, with Congress paralyzed into inaction by a fractured Republican majority in the House that is unable to pass the spending bills needed to keep federal agencies operating past Sept. 30. Complicating House Republican calculations is Mr. Trump, who has demanded that his followers vote against any spending measure that keeps funding the Justice Department’s prosecution of him over his efforts to overturn the 2020 election and hide highly classified documents that he took from the White House. It is an impossible request.

The seven candidates on the stage will almost certainly be asked their views. Their answers could prove to be a useful counterweight to Mr. Trump’s “SHUT IT DOWN!” instruction — or more fuel to drive Republicans toward an economically damaging and politically risky crisis that would dominate headlines for weeks.

What the candidates say about Ukraine

At the heart of the looming shutdown is a key foreign policy question: Should the United States continue its military aid to Ukrainian forces battling Russia’s invading army? The issue has divided Republicans in Congress and on the presidential campaign trail, elevating candidates like Mr. Ramaswamy and, to some extent, Mr. DeSantis, whose tepid support at best for more aid may appeal to isolationist voters who embrace Mr. Trump’s America First mantra.

Support for Ukraine has become a mark of traditional foreign policy conservatism, embraced most strongly by Mr. Pence and Ms. Haley. Will they stand by their pro-Ukraine positions or bend in the face of Republicans who are ready to shut down the government to stop any more taxpayer dollars from flowing to Kyiv?