Some of the stocks under attack belong to large SPACs that surged in recent months, in part because they were backed by high-profile financiers. A blank-check company created by venture capitalist

Chamath Palihapitiya

that plans to merge with lending startup Social Finance Inc. is a popular target, with 19% of its shares outstanding sold short, according to data from S&P Global Market Intelligence. The short interest in

Churchill Capital Corp. IV,

a SPAC created by former investment banker

Michael Klein

that is merging with electric-vehicle startup Lucid, more than doubled in March to about 5%.

Others are wagering against companies after they combine with SPACs. Muddy Waters Capital LLC announced last week it was betting against

XL Fleet Corp.

, a fleet electrification company that went public in December after merging with a SPAC. XL has since said Muddy Waters’s report, which alleged XL inflated its sales pipeline and made misleading claims about its technology among other issues, had “numerous inaccuracies.” 

XL’s stock price dropped the day Muddy Waters released its report by about 13%, to $13.86, from its prior close on March 2. Shares closed Friday at $12.79.

Shares of

Lordstown Motors Corp.

fell nearly 17% Friday after Hindenburg Research released a report saying the electric-truck startup had misled investors on its orders and production. The company, which merged with a SPAC in October, said the report contained half-truths and lies. The short interest in Lordstown shares rose to 5% from 3.4% in the week before the report’s publication, according to data from S&P.

“SPACs are an area of focus,” said Muddy Waters’s

Carson Block.

The veteran short seller said SPACs largely make up the universe of companies he views as both “abysmal” and relatively free from technical challenges, such as high short interest, which can make betting against them difficult.

SPACs are shell firms that raise capital by issuing stock with the sole purpose of buying or merging with a private company to take it public. They are dominating the market for new stock issues, becoming a status symbol for celebrities while pumping the value of acquisitions, like betting company

DraftKings Inc.,

into the tens of billions of dollars.

Hedge funds that buy into SPACs early see them as a way to make lofty returns without much risk. Individual investors are attracted by the chance to get positions in newly public companies that they could rarely purchase through traditional IPOs. The Securities and Exchange Commission issued a statement on Wednesday warning that it “is never a good idea to invest in a SPAC just because someone famous sponsors or invests in it.”

A monthslong rally in the stocks lost steam recently amid a broad selloff in technology and high-growth companies. An index of SPAC stocks operated by Indxx fell about 17% from mid-February to March 10, while the Nasdaq Composite Index declined about 7.3% over the same period.

“These are all momentum stocks, and a lot of people want to short them,” said

Matthew Tuttle,

whose firm Tuttle Tactical Management runs an exchange-traded fund that allows investors to hold a portfolio of SPAC stocks. Mr. Tuttle is preparing to launch an ETF that bets against “de-SPAC” stocks of companies that have merged with a SPAC—like electric-truck manufacturer

Nikola Corp.

and baked-goods maker

Hostess Brands Inc.

—and a separate fund that invests in the stocks.

Postmerger companies are particularly attractive to short because they have larger market capitalizations, making their shares easier to borrow, and because early investors in the SPACs are eager to sell shares to lock in profits, analysts and fund managers said.

Short sellers borrow stocks they believe are overvalued and immediately sell them, hoping to repurchase the shares for a lower price when they need to be returned and to pocket the difference. The strategy proved dangerous in recent months when individual investors organized on social media to push up stocks like GameStop Corp., forcing short sellers to buy shares and cap their losses, helping to drive prices still higher.

Continued strong investor demand for SPACs could catch short sellers in a similar squeeze. Shorting SPACs can also be risky because their shares have a natural floor at $10, the price at which they can be redeemed before a merger, and because they are prone to sharp price moves, analysts said.

Still, the portion of shares sold short in SPACs and their acquisitions is climbing.

Some are betting against stocks they believe rose too fast, to unsustainable valuations. The price of bioplastics company

Danimer Scientific Inc.

nearly tripled to $64 in the first six weeks of the year after it was bought by a SPAC. The short interest in Danimer stock has climbed to 8.5% from around 1% in January, and its share price has traded down to about $42, according to data from S&P.

Others are making bearish bets to hedge against potential losses in SPAC stocks they own.

Veteran short seller

Eduardo Marques

cited SPACs and their boosting the number of U.S.-listed stocks as a short-selling opportunity, according to a pitch for a stock-picking hedge fund called Pertento he plans to launch this year. America’s roster of public companies had shrunk from the mid-1990s onward, but that trend has recently reversed, partly because of SPACs.

Their popularity has helped spark new Wall Street offerings.

Goldman Sachs Group Inc.

this year started offering clients set baskets of similar stocks to short, pitching them as a way to hedge SPAC exposure, people who have seen the offering said. Clients typically customize the baskets Goldman offers, which are thematic and sector-focused, such as on bitcoin and electric vehicles.

Kerrisdale Capital founder

Sahm Adrangi

started shorting postmerger SPAC companies earlier than most, with a public bet in November against the stock of frozen-food maker

Tattooed Chef Inc.,

which still trades above its price at that time. But the stock has fallen about 13% during the recent market slump.

“We saw these stocks go up a lot and now that people are de-risking, these highflying SPACs are coming down to earth,” Mr. Adrangi said.

—Amrith Ramkumar and Mike DeStefano contributed to this article.

Write to Matt Wirz at matthieu.wirz@wsj.com and Juliet Chung at juliet.chung@wsj.com