WASHINGTON — President Trump endorsed a proposal on Monday to privatize air traffic control, seizing on a decades-old idea as proof that he is advancing the ambitious infrastructure rebuilding plan he promised during his campaign but is still months from delivering.

At an East Room event that was choreographed like the elaborate ceremonies for enacting major legislation, Mr. Trump signed a memo and letter to Congress outlining his principles for overhauling the nation’s air traffic control system. He handed out pens to lawmakers who had been invited to attend, and reveled in several rounds of applause.

But Mr. Trump’s announcement did not have any binding effect, and Democrats quickly denounced the proposal.

He did not take any action on Monday toward a broader $1 trillion initiative to rebuild the nation’s crumbling infrastructure, including old and inefficient airports, a subject he spoke about many times as a candidate. His advisers say that package is months away, though Mr. Trump said in April that it would be ready last month.