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

It was the third time in three weeks that Mr. Trump has directed the government to target a perceived political enemy. He has drastically expanded the use of his powers to try to damage domestic opponents, eroding a post-Watergate norm of Justice Department independence far more than he ever did in his first term.

Mr. Trump called 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 asked for a report on the results of Ms. Bondi’s investigation within 180 days.

Mr. Trump’s 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 had 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.

Just over two weeks ago, Mr. Trump signed memorandums targeting two officials from his first term for investigation. 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.

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