WASHINGTON – President Donald Trump said Sen. Bernie Sanders, who entered the growing field running for the 2020 Democratic presidential nomination on Tuesday, had "missed his time" for a successful bid for the White House.

Trump has repeatedly raised what he has been describing as the threat of socialism in U.S. politics, most recently during a speech Monday directed at Venezuelan-Americans. Many have viewed Trump's remarks as at least partly aimed at a number of progressive Democrats running to unseat him in 2020, including Sanders.

"Personally, I think he missed his time," Trump told reporters in the Oval Office on Tuesday. "I wish Bernie well. It will be interesting to see how he does."

Sanders, an independent from Vermont, sent a campaign email Tuesday announcing that he will once again seek the Democratic nomination, jumping into an already crowded field of nearly a dozen candidates, with many more expected to join. Sanders lost to Hillary Clinton in 2016, but won wide support in the Democratic party's liberal wing.

"I’m running for president because, now more than ever, we need leadership that brings us together – not divides us up," Sanders wrote in the email. "Women and men, black, white, Latino, Native American, Asian American, gay and straight, young and old, native born and immigrant. Now is the time for us to stand together."

Sanders described himself a democratic socialist and focused on social and economic justice, a platform seen as too radical by many in the Democratic Party establishment.

Trump spoke to reporters Tuesday after signing a policy directive to create his long-promised Space Force, an arm of the military. He also took a swipe at former Texas Rep. Beto O'Rourke, who ran an unsuccessful campaign for Senate in Texas last year and who is considering a run for president in 2020, suggesting the Democrat's comments opposing a border wall were "probably the end of his political career."

"You have a lot of people running. But only one person is going to win," Trump said. "I hope you know who that person is.”

Contributing: Aki Soga, Burlington Free Press