Donald J. Trump didn’t wait for the opening bell before blaming Monday’s market sell-off on Vice President Kamala Harris.

“Stock markets are crashing, jobs numbers are terrible, we are heading to World War III, and we have two of the most incompetent ‘leaders’ in history,” the former president and Republican presidential nominee wrote in a post on Truth Social at 8:12 a.m. Eastern time. “This is not good.”

Mr. Trump did not mention that markets had suffered far greater single-day losses when he was president, or that economists blamed a variety of factors — including a disappointing July jobs report, a plunge in Japanese markets earlier in the day and a growing consensus among investors that the Federal Reserve has waited too long to start cutting interest rates — for Monday’s slide.

He also did not mention that earlier this year, he had claimed credit for a surge in stock prices, which he said reflected confidence he would be re-elected.

What Mr. Trump was engaged in was a calculated attempt at political marketing. By 9:45 a.m. on Monday, less than an hour after U.S. markets opened, Mr. Trump branded what would become a 3 percent decline for the day in the S&P 500 the “Kamala Crash.”

By lunchtime, it was official party messaging: The Republican National Committee hyped the “Great Kamala Crash of 2024,” and the Trump campaign had produced and circulated on social media a video tying the vice president to Monday’s dip in the markets. By the afternoon, the Trump forces had turned “KamalaCrash” into a “trending” subject on X.

The coordinated effort underscored Mr. Trump’s longstanding fixation on stock indexes as a barometer of economic health and even as a substitute for polls — a measure of his own performance and popularity. Yet markets can rise and fall sharply from day to day; Japan’s stock exchange surged in early trading on Tuesday after big losses on Monday.

It also reinforced the degree to which economic messaging, and the health of the economy itself, will play a key role in the sprint finish ahead of the presidential vote in November.

“What middle-class families need is steady economic stewardship, not chaotic ranting lies,” Ammar Moussa, a Harris campaign spokesman, responded in a statement. “Donald Trump had the worst jobs record of any modern president, and oversaw some of the worst days in the stock market in history while spending his presidency lining the pockets of his wealthy friends who shipped American jobs overseas. Economic experts agree: His plans would raise costs on working families by $2,500 a year and ‘supercharge inflation.’”

Ms. Harris has stressed economic optimism in her speeches. She portrays the economy as strong and tries to cast Mr. Trump’s policy proposals, like new taxes on imported goods, as a threat to growth and a possible spark of recession.

“We believe in a future that keeps America’s economy the strongest in the world,” she said in Houston last week. “Where every person has the opportunity to build a business, to own a home, to build intergenerational wealth.”

American voters consistently tell pollsters that the economy and consumer prices are the most important issues facing the country. Mr. Trump and his presumptive Democratic opponent, Ms. Harris, are seeking to sell voters on diametrically opposite stories about the economy’s health.

Mr. Trump wants voters to believe that the economy is on the brink of catastrophe and that Ms. Harris and President Biden are to blame. He has leaned into a negative view of the economy that until now has largely focused on the nation’s prolonged bout of high inflation earlier in Mr. Biden’s term.

Polls have consistently shown that a majority of Americans believe the economy is in recession, even though economic statistics suggest it is not, a fact that vexed Mr. Biden as he tried to campaign on the actual record. Economic growth was surprisingly strong in the first half of the year. Job growth has remained relatively strong, even with the slowdown in job creation in July. Inflation has finally fallen to more normal historical rates.

None of that data has done much to change voter concerns. And it has not stopped Republicans from warning repeatedly in the last few years that the economy was tipping into a downturn. Monday was no different.

“This moment could set off a real economic calamity around the globe,” Senator JD Vance of Ohio, Mr. Trump’s running mate, posted on X on Monday. “It requires steady leadership — the kind President Trump delivered for four years.”

(Mr. Trump’s presidency included a rapid descent into a pandemic recession in 2020, including a steep drop in the stock market that was followed by a rebound that summer.)

Mr. Trump has set up this moment as a heads-he-wins, tails-she-loses scenario. He took credit for the stock market’s double-digit gains earlier this year — claiming it was booming because investors thought he would be president next year. Now that global markets are falling, Mr. Trump is claiming it’s the fault of Biden-Harris policies.

Economists and analysts dismissed Mr. Trump’s stock-market boasts earlier this year and his critique of Ms. Harris on Monday.

“Typically presidents take credit when the market is up and are probably unfairly blamed when the market is down,” said Chris Krueger, who works at the research group TD Cowen.

Mr. Krueger pointed to the plunging Nikkei, which fell 12.4 percent on Monday, its biggest decline since the Black Monday crash of October 1987. “How any U.S. politician bears responsibility for that is a bit mystifying, but that’s sort of table stakes for where we are in the 2024 election,” he said in an interview on Monday.

Goldman Sachs economists raised their odds of a recession in the next year on Monday to 25 percent, from 15 percent. Alec Phillips, Goldman’s chief political economist, said there was little evidence the sell-off was hurting Ms. Harris right away, citing election prediction markets.

“Harris’s odds in prediction markets didn’t move following the jobs report on Friday, and are still at basically the same level now,” Mr. Phillips said, “so at least the consensus view so far is that these numbers alone don’t have much bearing on the election outlook.”

The Federal Reserve could still affect the outlook, though. If officials at the central bank cut interest rates in September, they could help to push down borrowing costs for Americans buying homes, cars and other big-ticket items on credit — a move that White House economists have long believed could help reinforce the idea that inflation is under control and that the outlook for consumers is improving. That could help Ms. Harris, especially if the Fed makes a larger-than-expected rate cut in a bid to jolt economic activity.