It's not every day that the president of the United States faces an impeachment. In fact, it's so rare that an American president has never successfully been removed from office by way of impeachment. Even still, that precedent — or lack thereof — hasn't stopped people from betting on Trump's chances of being impeached. As it turns out, the odds appear to be getting better.

Per reporting from MarketWatch this week, the gambling odds of Trump being impeached have risen since reports that the president allegedly asked Comey to end an FBI investigation into Michael Flynn surfaced on Tuesday. For its part, the White House denied the allegation, saying in a statement on Tuesday that "the President has never asked Mr. Comey or anyone else to end any investigation." Still, some European betting organizations have Trump's odds of impeachment at 2 to 1 — or 33 percent — following an increase in bets after Tuesday's news. Others placed the odds at 27 percent. According to a report from Fortune, some say it's as high as a 60 percent chance that Trump will leave office early.

Betting on Trump's impeachment has been available, at least in Europe, as early as his election last November, according to the MarketWatch report. Even before this week's allegations of obstruction of justice, the odds of Trump's impeachment were reportedly increasing in the gambling realm, jumping from 16 to 1 after the election in November to 10 to 1 by the inauguration in January.

Mark Wilson/Getty Images News/Getty Images

Typically, betting odds are determined by a team of experts based on the probability of an event occurring and the probability that money will be placed on a certain outcome. In other words, the higher odds of Trump's impeachment, as reported by these different betting organizations, could be, at least partially, a result of more people betting on an impeachment to occur.

The uptick in bets is most likely a result of this week's news that Trump may have committed obstruction of justice, widely considered an impeachable offense. Again, the White House has denied the allegations that Trump asked Comey to end an investigation. But impeachment rumors began to swirl nonetheless after the existence of Comey's memo on Trump's alleged request became public. Shortly thereafter, those rumors were further fueled when the House Oversight Committee, the chamber's investigative body, requested all records of conversations between Trump and Comey from the FBI. On top of that, a special counsel was appointed to take over. An investigation tends to be one of the first steps in the impeachment process.

Drew Angerer/Getty Images News/Getty Images

During a news conference on Thursday, Trump criticized the investigation and firmly denied any ties between his campaign and Russia. "The entire thing has been a witch hunt and there is no collusion between certainly myself and my campaign — but I can always speak for myself — and the Russians," he said. "Zero."

Ultimately, the true chances of Trump's impeachment all come down to Congress. The House of Representatives must vote to charge Trump with one or more articles of impeachment. Then, the Senate must conduct a trial to determine if he is guilty and, therefore, removed from office. Given the Republican majorities in both houses of Congress — and the slow-moving nature of the federal government — the president may have some time before any impeachment proceedings come to fruition.