Two of America’s most popular online betting markets are now favoring Vice President Harris to win the general election this year over President Trump, a first since she announced her candidacy and a sign of grassroots optimism that largely eluded President Biden. The markets may have been boosted by new analysis that finds critical Sun Belt states are now back in play for the Democrats. 

Obsession with two online betting forums — Polymarket and PredictIt — began in earnest after Mr. Biden’s disastrous debate performance with Trump in June. Users were betting like mad on questions such as who would win the presidency in November, who would be the Democratic nominees for president or vice president in the wake of the debate, and who was favored to win the popular vote or the Electoral College, or both.

Hundreds of millions of dollars have been spent so far betting on those questions. On Polymarket’s “who will win the presidency” question alone, more than $550 million has been wagered. 

Ms. Harris and Trump are now tied at 49 percent each in the Polymarket betting. On the day Mr. Biden announced he would drop out of the race, just 29 percent of bettors thought Mr. Harris would prevail, compared to 63 percent who said the same of Trump. 

The gap between the Democrats and Trump in Polymarket’s betting system grew steadily over the course of months, and then shifted rapidly after the debate performance. Trump had about a 20-point lead in betting markets over Mr. Biden beginning in May, and it then opened to a more than 40-point lead after the CNN debate. Ms. Harris is overwhelmingly favored to win the popular vote, at least, with a 70 percent likelihood, according to the Polymarket betting. 

She has seen a 4 percent increase in the likelihood she will win since she announced Governor Walz as her running mate on Tuesday and the two kicked off a multi-state tour. 

In critical battleground states, the race is tied according to bettors. Polymarket users consider Nevada, Wisconsin, Pennsylvania, and Georgia to be the four most competitive states, while Trump is slightly favored in Arizona and North Carolina.  

PredictIt also favors Ms. Harris to win the presidency in November, though users of that platform are more sure of a victory for the vice president than are Polymarket bettors. Ms. Harris leads Trump by 9 points on the PredictIt market’s question of who will assume the presidency in January. 

The volume of bets shot up around the time Mr. Biden and Trump debated in June. The day before the debate, $26,400 was bet on the “who will win.” On the day of the debate, more than $318,000 was sent into the betting market. The day Mr. Biden dropped out was the highest spending day so far of the last three months, with nearly $700,000 being bet on users’ preferred candidates. 

Ms. Harris trailed Trump in the PredictIt markets until July 31, when both candidates hit a 49 percent odds of winning. Ms. Harris opened her wider nine-point lead after announcing Mr. Walz as her running mate.  

The vice president’s odds in the betting markets are likely buoyed by new polling and analysis showing that she is far more competitive in critical Sun Belt states than Mr. Biden ever was. Toward the end of Mr. Biden’s bid, his staff was shifting its focus to three “blue wall” states — Pennsylvania, Michigan, and Wisconsin — for a path to victory, though polling from those states showed Trump running away with an Electoral College victory. 

Ms. Harris has easily secured her place as a competitive — if not favored — presidential candidate. Her ability to potentially expand the map in North Carolina, Georgia, Nevada, and Arizona have not gone unnoticed by political bettors. 

On Thursday, the Cook Political Report moved Arizona, Nevada, and Georgia to “toss up” from leaning toward Trump in the general election. 

“Three weeks ago, Donald Trump was leading President Joe Biden in the Cook Political Report national vote tracker by about 2.5 percent. Today, Kamala Harris leads Trump by less than one point, a shift of more than three points in Harris’ direction,” the report’s publisher, Amy Walter, writes. “Battleground state polls are showing similar results, with Harris cutting into Trump’s lead by anywhere from two to five points.”