President Trump on Thursday plans to direct the Justice Department to investigate ActBlue, the fund-raising platform that powers virtually every Democratic candidate and cause, according to a person briefed on the preparations. The move steps up Republicans’ effort to cripple their opponents’ political infrastructure.

It will be the third time in three weeks that Mr. Trump has directed the government to target a perceived political enemy, a drastic expansion of his use of his powers to try to damage domestic opponents.

Mr. Trump plans to call for an investigation by Attorney General Pam Bondi into ActBlue, which is used across the Democratic Party’s ecosystem to collect donations online. The inquiry is ostensibly meant to look into possible illegal donations made by people in someone else’s name, known as straw donations, as well as hard-dollar contributions from foreign donors.

Mr. Trump’s impending action represents a threat to one of the key financial cogs of the left, potentially hindering Democrats’ ability to compete in elections. It is likely to please elements of his base, for whom ActBlue has become a top target. Congressional Republicans have separately been investigating what they claim are the platform’s insufficient security provisions.

For days, Democratic groups have been worried that the White House was planning executive orders or memorandums that would target an array of nonprofit organizations. White House officials insisted no such orders were being drafted and maintained that stance for days.

On Thursday, Politico and other news outlets reported that Mr. Trump planned to sign an ActBlue memorandum later in the day. The person briefed on the preparations insisted that the memorandum was different from the type of order that Democrats had speculated might be in the offing, because it related to foreign donations.

Just over two weeks ago, Mr. Trump signed memorandums targeting two officials from his first term for investigation by his current government. One, Miles Taylor, has been deeply critical of the president. The other, Chris Krebs, was targeted for rejecting Mr. Trump’s false claims of widespread election fraud involving voting machines.

The Republican scrutiny of ActBlue has focused on claims — thus far unsubstantiated — that it allows straw and foreign donations. Federal election law bars straw donations, and it prohibits foreign citizens without permanent residency from donating directly to federal political candidates or political action committees.

A Justice Department investigation into ActBlue is likely to create vulnerabilities for the entire Democratic fund-raising apparatus. Party consultants have relied on ActBlue to bring in donations. Candidates, committees for federal and state legislative chambers, and liberal caucuses use the platform as their primary mechanism to process donations.

“ActBlue plays a vital role in enabling all Americans to participate in our democracy and the organization strictly abides by all federal and state laws governing its activities,” said Megan Hughes, an ActBlue spokeswoman. “We will always stand steadfast in defending the rights of all Americans to participate in our democracy and ActBlue will continue its mission undeterred and uninterrupted, providing a safe, secure fund-raising platform for the millions of grass-roots donors who rely on us.”

There is great fear across the Democratic Party that any of the entities that have used ActBlue could soon find themselves enmeshed in an investigation into foreign contributions from a hostile Justice Department with direction from Mr. Trump.

As word of the impending Trump memorandum circulated among Democrats, panicked Democrats blasted fund-raising appeals.

“Please, while we still can, make a donation to my campaign’s emergency fund through ActBlue,” Senator Ruben Gallego of Arizona wrote to supporters on Thursday afternoon. “Any amount at all. We’ve got to be ready for any outcome, and we’ve got to start preparing now.”

ActBlue itself got into the fund-raising game. Regina Wallace-Jones, the platform’s chief executive, wrote to Democratic partners late Wednesday asking for money to help “fight against the creeping despotism of the right, and to win back power in Washington, D.C., and the halls of government across the country.”

She wrote that the looming threat of an executive order or memorandum from Mr. Trump had already damaged ActBlue and its allies.

“The current strategy of distraction and exhaustion is effective,” she wrote. “We see this across the country and are not immune to this ourselves. The flow-on effect from the initial innuendo of the E.O. caused many in the ecosystem anxiety and distress.”

ActBlue has faced internal turmoil since Mr. Trump won the presidential election in November. At least seven senior officials quit the organization in late February, prompting two employee unions to warn of an “alarming pattern” of departures that was “eroding our confidence in the stability of the organization.”

Republicans have been encouraging the Trump administration to investigate ActBlue.

This month, the leaders of three Republican-led House committees accused the group of not doing enough to prevent fraud and demanded more information about the recent resignations. Last week, several of the people who left ActBlue received their own letters asking them to appear before Republican congressional investigators.

Last month, several Republican lawmakers urged Treasury Secretary Scott Bessent to investigate ActBlue or to help them do so.

And Representative Andy Biggs of Arizona asked the F.B.I. to investigate whether ActBlue had allowed Democrats “to skirt the integrity of federal campaign finance laws,” including by processing donations that originated in hostile foreign countries.

Elon Musk, the president’s billionaire adviser who poured hundreds of millions of dollars into the 2024 election, has criticized ActBlue for weeks, claiming without evidence that the organization was funded by Democratic megadonors including Herb Sandler, who died in 2019. On Thursday, Mr. Musk wrote on his social media platform, “ActBlue is guilty of widespread criminal identity theft.”

Last month, the White House brought in Scott Walter, the president of the conservative watchdog group Capital Research Center, which has investigated ActBlue, to brief senior officials on the organization and other aspects of Democratic political financing.

On Thursday, Mr. Walter suggested that the planned memorandum was about compliance with election law, and was not an effort to undermine Democrats’ electoral prospects.

“Liberal funding schemes for political and charitable giving have drawn criticism from left- and right-leaning watchdogs,” he said in a statement.

Democrats and their allies reject that.

In a fund-raising email on Thursday, the Democratic-aligned news organization Courier Newsroom said the actions against ActBlue and other liberal groups “aren’t about election security — they’re about silencing dissent and cutting off the resources behind grass-roots resistance to Trump’s authoritarianism.”