WASHINGTON — President Donald Trump currently faces an impeachment probe over his private and public calls, despite no existing evidence of any crime, for foreign investigations into former Vice President Joe Biden and his family.

But Biden's Democratic rivals know that while it's the former VP who has Trump’s attention today, there’s no telling who it might be tomorrow: the president has such a long history of making unfounded charges about opponents that any of the other Democratic presidential candidates could be the next to be tarred with a conspiracy theory.

In the past week, Trump has called for the Democratic chairman of the House Intelligence Committee to be arrested “for treason” for paraphrasing a readout of his Ukraine call in a hearing instead of quoting it — treason being a capital charge with no relevance to their dispute.

Last month, for no clear reason, he called for investigations into former President Barack Obama’s post-White House deals with Netflix and his book publisher. Earlier this year, he made a baseless demand for an investigation into Rep. Elijah Cummings, D-Md.

Previously, Trump claimed he personally funded investigations into Obama’s birth certificate and publicly called on hackers to find his college records. He pushed a National Enquirer story falsely alleging Sen. Ted Cruz’s father was involved in the John F. Kennedy assassination and demanded the press investigate further.

Trump’s long record of similar accusations — and his apparently heightened interest in pushing his administration to advance them — are one reason the impeachment inquiry has taken on such urgency for Democrats, some of whom fear that what Biden faces now could descend on any number of 2020 contenders should they get close to the nomination.

“Every single candidate in this race is going to have their time in the barrel, that is the way that Trump and his allies work,” said Zac Petkanas, a Democratic strategist who ran the Hillary Clinton campaign’s rapid response team. “His ability and willingness to manipulate the levers of power to his personal and political benefit is a huge concern going into the 2020 election.”

Petkanas recommended Democrats develop a careful plan to debunk attacks in broad terms without repeating the details, a tactic Biden has employed at times by broadly citing outside sources who’ve found Trump’s allegations against him baseless.

“The intent of misinformation is not to prove to people whether it’s true or not, it’s to create a debate around the issue,” he said.

Republican allies of Trump, including Vice President Mike Pence, have argued Biden’s established family entanglements with Ukraine — namely his son’s work for a Ukrainian energy company — deserve to be scrutinized.

But the particulars of the Biden case, while not unimportant on their own, are largely irrelevant to the question of whether it’s appropriate or legal to push for investigations, at home or abroad, into political rivals. The lack of any incriminating evidence behind the calls for an investigation into Biden and the timing in the election cycle also raise the specter that the president could seize on thin or false claims to target other candidates.

There will be no shortage of fringe figures looking to push conspiracy theories into Trump’s orbit that could provide grist for future calls for such probes.

This week, a far-right pro-Trump personality facing a felony indictment sought to spread a baseless claim about Sen. Elizabeth Warren, D-Mass., after being caught spreading an easily debunked smear against South Bend, Indiana, Mayor Pete Buttigieg earlier in the race.

Warren laughed it off with a tweet that indirectly referred to the story, but there’s a history of similar figures sending claims upwards from online message boards to the president himself.

Full coverage: Trump impeachment inquiry

“Within 24 to 48 hours, something can push from 4chan or Reddit to individuals on Twitter, then they end up putting them on TV,” Benjamin T. Decker, a researcher who specializes in disinformation and the founder of digital investigations firm Memetica, said. “In that regard, it’s easy to hack Trump’s attention, because it’s very clear what his media consumption points are.”

This dynamic has already played out in the current scandal. In the same call with a Ukrainian leader in which Trump pressed for an investigation into Biden, he called for a separate probe into what seem to be a confused web of false conspiracy theories that migrated from the online message board 4chan to Fox News hosts, intended to absolve Russia of the 2016 Democratic National Committee hack.

Trump’s former Homeland Security adviser, Tom Bossert, expressed concern this week that the president and his aides appeared fixated on these “completely debunked” series of claims.

As an example of how this phenomenon might end up targeting a political rival, Decker pointed to last month, when a video falsely claiming to show Rep. Ilhan Omar, D-Minn., celebrating on the anniversary of 9/11 moved through pro-Trump users on Twitter and into the president’s own feed.

As if to illustrate the point, Trump personally feted the supporter who had spread the false Omar story and another conspiracy theory the president had retweeted about Bill Clinton at a White House event on Friday, inviting him to speak.

Rick Tyler, an MSNBC analyst who worked on Cruz’s presidential run, said that the campaign was caught off guard by Trump’s infamous JFK smear in part because they assumed it would be treated as “ridiculous” by voters.

This time, with Trump’s words carrying the power of the presidency and a well-established pattern of behavior, Democratic campaigns are less likely to be surprised if they face tactics similar to what’s befallen Biden.

“It’s like watching from the window while a crane swings a wrecking ball in the opposite direction of a building you’re in,” Tyler said. “We don’t have to wonder whether the wrecking ball is going to hit the building. It’s going to hit the building.”