He is rising in the polls and turning heads in Iowa and New Hampshire, behind heavy spending on ads that play to voters’ appetite for a leader who is upbeat and positive in a dark political moment.

He has experience, a compelling personal story and a campaign war chest that gives him staying power in a Republican primary that so far has been a two-man race. And among Republican voters, he is the candidate that everyone seems to like.

Senator Tim Scott of South Carolina is perfectly positioned to seize the moment if former President Donald Trump collapses under the weight of his criminal cases or if the challenge to him from Gov. Ron DeSantis of Florida evaporates.

The only question is whether either moment will come.

Mr. Scott’s growing popularity in early primary states has made him more of a contender in the still-young primary campaign and — in the eyes of current and potential supporters, and donors — a possible alternative to Mr. DeSantis, who is seen as an alternative to Mr. Trump.

Andy Sabin, a wealthy metals magnate who switched his allegiance from Mr. DeSantis to Mr. Scott and is hosting a fund-raiser for three dozen wealthy donors in the Hamptons next month, cited his frustration with the front-runners and said he hoped that more in the donor class would join him in backing Mr. Scott. Prospective donors, Mr. Sabin said, “all want to see what he’s about.”

“They’re disenchanted with Trump and DeSantis,” he said. “And the others, I’ve seen very little momentum.”

Since he entered the race in May, Mr. Scott’s standing has slowly crept up in Iowa and New Hampshire. A University of New Hampshire poll of likely voters, out Tuesday, found him in third place among the state’s primary voters, with 8 percent of the vote, ahead of former Gov. Chris Christie of New Jersey and former Gov. Nikki Haley of South Carolina, both of whom have focused intensely on the state.

He is also running third in recent Iowa polls — at around 7 percent — and a few national polls have shown him as the second choice for many supporters of Mr. Trump or Mr. DeSantis, though it comes at a time when primary voters not committing to Mr. Trump are often considering several candidates.

Mr. Scott’s strength in early states has caught the eye of other potential donors, including the billionaire cosmetics heir Ronald Lauder, who met with Mr. Scott in South Carolina this month. In August, Mr. Scott will make a fund-raising swing through at least five states, including Colorado, Tennessee and Wisconsin.

While he has not been as much of a presence on the campaign trail as his rivals have, Mr. Scott and his allied groups have poured considerable money into Iowa and New Hampshire, spending $32 million to run ads through January 2024 — more than any other Republican candidate or group on the airwaves, according to the tracking firm AdImpact. Mr. Scott is the only Republican contender who has booked ad time that far ahead.

Mr. Scott’s supporters say his positive campaign message and general appeal provide a contrast with the primary’s front-runners. The highest-ranking Black Republican, he is running on an only-in-America story as a candidate and a senator with roots in a low-income Charleston community.

Still, though Mr. Scott has shown some momentum in the early states — including his home state — Republican voters have yet to flock to him en masse, and he is still relatively unknown nationally.

A Quinnipiac University poll of voters nationwide found him tied with Mr. Christie in the primary among likely Republican voters, behind Ms. Haley and former Vice President Mike Pence, who are tied for third. And while he is well-liked in early primary states, more than half of Republican voters surveyed nationally said they did not know enough about him to have an opinion.

Alex Stroman, the former executive director of the South Carolina Republican Party, acknowledged the issue but said that it was solvable. “I think that the more people are introduced to Tim Scott, that they are going to like Tim Scott,” he said. “The problem is, it is a crowded primary.”

Asked during a town hall in New Hampshire on Tuesday how voters should contend with such a crowded field, Mr. Scott said he expected that “the field will dwindle pretty quickly” by the time voters cast ballots in the state’s February primary election.

The first opportunity to introduce himself to a national audience will be the Aug. 23 Republican debate. Mr. Scott’s campaign manager, Jennifer DeCasper, said recently that Mr. Scott had met the donor and polling thresholds to be on the debate stage. Mr. Scott, who raised more than $6 million in the second quarter, has more than $20 million in the bank — one of the largest war chests in the primary and enough, Ms. DeCasper maintained, to keep his campaign afloat through the Iowa caucuses and all three of the early state primaries.

“At the end of the day, candidates can post any number they want,” she said. “But the name of the game is how much actual cash you have on hand that’s available for use in the Republican primary.”

On Tuesday, Trust in the Mission PAC, a group supporting Mr. Scott, announced that it would spend $40 million on broadcast and digital advertising in Iowa, New Hampshire and South Carolina — a gigantic outlay that far outpaces the spending of any other candidate in the G.O.P. field and could possibly reshape it.

The PAC’s spending reflects a huge bet on increasing Mr. Scott’s profile, especially as he maintains a relatively limited presence on the campaign trail: He has relegated his time in early primary states this month to the few days of the week that he is not in the Senate. The group has already shelled out more than $7 million on advertisements through the summer; the $40 million buy will kick in beginning in September. It is also helping fund a small field operation of about a dozen canvassers in the early primary states.

One challenge Mr. Scott still faces is presenting a policy message that separates him from the rest of the Republican primary field. His advertisements in Iowa, New Hampshire and South Carolina are biographical, and some touch on national security, warning of the threat that China could pose, while others seize on cultural issues, criticizing Democrats’ policies on education and their views on race.

But trying to appeal to a broad swath of Republican voters without alienating key portions of the party’s primary electorate has proved challenging.

Terry Amann, an Iowa pastor who has met with most of the Republican candidates, said Mr. Scott needed to articulate a more solid policy plan to connect with the conservative evangelicals who could decide the caucuses in January. Though the senator’s conservative message and his frequent biblical allusions have endeared him to many Republican faith-based voters, Mr. Amann said, Mr. Scott has not clearly defined his stance on abortion restrictions.

“If you’re going to be the candidate that stands out on faith, there are some issues that I believe are worth laying it down for, and that’s one of them,” he said. “That would be my challenge to him if he wants to step off from the rest of the pack.”

With just over a month until the first debate and six months until the Iowa caucuses, Mr. Scott’s campaign still sees an opening to refine his message and consolidate more voters. Still, while he tries to surpass Mr. DeSantis, the bigger challenge will be wresting the support of more than half of Republican primary voters from Mr. Trump.

“These campaigns, candidates, have to figure out what the hell they want voters to know about them,” said Dave Carney, a veteran Republican strategist in New Hampshire.

Mr. Scott, because of his background, has a unique story to tell, which can get “people to listen a little bit,” Mr. Carney said. “That’s a great advantage.”

But, he added, “the point isn’t just to get their interest — then you have to make the deal.

”You have to sell the deal.”

Ruth Igielnik contributed reporting.