President Donald Trump's shock firing of James Comey has brought the topic of impeachment to the fore.

The possibility of the president being impeached by the House of Representatives and convicted by the U.S. Senate seems unlikely, seeing as Trump's fellow Republicans hold majorities in both chambers and Trump's popularity remains high among GOP voters.

But the sacking of Comey, who as FBI director was leading the most serious official investigation into possible collusion between the 2016 Trump campaign and Russian hackers seeking to influence the election, has changed the calculus for bettors.

"In the past 24 hours alone we've seen money for Trump to be impeached in his first term, resulting in us cutting the odds from 10/11 into 4/6," Irish bookmaking firm Paddy Power said in The Independent on Thursday. "We can attribute this to the news of Comey's sacking. [That] is the shortest we've been for Trump to be impeached in his first term."

Those odds put the possibility of a Trump impeachment at about 60 percent.

Why would Trump's removal of the FBI director -- who serves at the pleasure of the president -- shift the odds so dramatically? Because Trump's actions raise questions about obstruction of justice, a charge that played a featured role in the articles of impeachment that President Richard Nixon faced 40 years ago. Nixon's efforts to head off the Watergate scandal, the House Judiciary Committee's impeachment resolution stated, led to him "interfering or endeavoring to interfere with the conduct of investigations by the Department of Justice of the United States, the Federal Bureau of Investigation, the office of Watergate Special Prosecution Force, and Congressional Committees."

Nixon resigned in 1974 after senators from his own party told him he had lost their support.

In the days since the Tuesday firing of Comey, Trump has continued to act in ways that are being widely interpreted as attempts to influence the Trump-Russia investigation.

In a Friday morning tweet, the president appeared to threaten Comey. Trump has claimed that the former FBI director had told him that he, Trump, personally wasn't under investigation, a claim Comey has not backed. Trump's tweet, perhaps inadvertently, offers echoes of the Watergate scandal, which ultimately turned on secret tapes made of Oval Office conversations.

James Comey better hope that there are no "tapes" of our conversations before he starts leaking to the press! — Donald J. Trump (@realDonaldTrump) May 12, 2017

Of course, impeachment isn't the only possible way Trump might leave office early. British bookmaker Bet365 has 11/4 odds that Trump will be removed in his first term through the invocation of the 25th amendment.

The amendment to the U.S. Constitution provides for a majority of the administration's cabinet "or of such other body" created by Congress to remove the president if they determine he is unable to properly discharge his duties.

-- Douglas Perry