President Trump’s executive order to defund NPR and PBS was met with fiery pushback on Friday, as the organizations challenged the legality of the move and said it could jeopardize access to vital information.

The order issued late Thursday night instructed the Corporation for Public Broadcasting, which receives and distributes over $500 million in taxpayer money to public TV and radio stations annually, to eliminate millions of dollars in federal funding to the two public media organizations. It amounts to perhaps the most significant threat in a decades-long campaign by Republicans to weaken NPR and PBS.

Patricia Harrison, the chief executive of the Corporation for Public Broadcasting, a private company, said in a statement that the White House had no legal authority over the company. NPR vowed to challenge the order, calling it “an affront to the First Amendment.”

Paula Kerger, the chief executive of PBS, also called Mr. Trump’s executive order illegal. “The president’s blatantly unlawful executive order, issued in the middle of the night, threatens our ability to serve the American public with educational programming, as we have for the past 50-plus years,” Ms. Kerger said.

Mr. Trump and other Republicans have long argued that NPR and PBS have a liberal bias and that taxpayers should not fund their journalism as a result. The executive order used the same argument, accusing the news outlets of producing “left-wing propaganda.”

Mr. Trump’s executive order was the fourth effort by Republicans to weaken public media in as many months: A bill is working its way through Congress to defund NPR and PBS; the White House asked Congress on Friday to rescind federal funding for the Corporation for Public Broadcasting; and this week, Mr. Trump sought to fire three directors from the Corporation for Public Broadcasting, a move that was delayed by the courts.