Americans learned Tuesday that President Donald Trump’s campaign aides allegedly communicated with Russian intelligence before he was elected, with contact being made around the same time Russia hacked into the Democratic National Convention’s email server. Several points about the communications remained unclear Wednesday: The general public still didn't know what was discussed or how close the ties between the Trump campaign and Russia actually were.

But the news motivated the president’s opponents to again demand his impeachment. As of Wednesday afternoon, more than 850,000 people have signed a petition to impeach the president, raising questions about whether Trump can impeached — and how exactly it would work.

Here's what you need to know.

Donald Trump President Donald Trump, then a presidential candidate, speaks at a rally in Dallas, Texas, Sept. 14, 2015. Photo: REUTERS/Mike Stone

The House of Representatives has the power to impeach the president, while the Senate has the power of court in impeachment trials. A president can be impeached if he commits offenses of “misconduct of public men, or, in other words, from the abuse or violation of some public trust,” wrote founding father Alexander Hamilton in the Federalist Papers in 1788.

In all of U.S. history, only two presidents have been impeached: Bill Clinton in 1998 after he lied about having an affair with a White House intern, and Andrew Johnson in 1868 for violating the Tenure of Office Act. Both presidents were acquitted. President Richard Nixon resigned before he could be impeached in 1974 after his administration was caught illegally meddling in the Democratic National Committee.

Trump has advantages over Nixon, Clinton and Johnson: No news outlet or intelligence agency has yet released proof that his campaign colluded with the Russians, for one. Plus, both the House and the Senate are dominated by Republicans, his own party. And although Trump isn’t an establishment Republican, traditionalists such as House Speaker Paul Ryan, R-Wisc., have been slow to challenge Trump.

But Democrats are demanding further investigations, columnists have been writing essays to try and persuade Congress to impeach Trump, and even Republicans are beginning to call for Trump’s impeachment.

“The leader of the band of Mad Hatters occupying the White House has already insulted allied world leaders, issued illegal and badly written orders, impugned a 'so-called' judge appointed by his own party, and appointed the least-qualified cabinet ever,” Mark Painter, a lifelong Republican and former Ohio judge, wrote in the Cincinnati Enquirer Tuesday.

“It’s tough, but we must end this dangerous presidency,” he added. “Trump must be impeached and removed with all haste. But only Congress can initiate the process.”