From the presidential announcements and fundraising reports to the debates, conflicts and departures, here's a look at the key events that have helped define and shape the contest heading into the Feb. 3 Iowa caucuses.
LOOKING BACK: A 2020 CAMPAIGN TIMELINE
Nov. 6, 2018: In the midterm elections, Democrats win control of the U.S. House of Representatives, while Republicans maintain control of the U.S. Senate.
Dec. 31: Sen. Elizabeth Warren, D-Mass., forms a presidential exploratory committee.
Jan. 3, 2019: The 116th Congress begins, with Democrats in control of the House.
Jan. 12: Warren formally announces her campaign.
Jan. 21: Sen. Kamala Harris, D-Calif., announces he presidential bid.
Jan. 23: South Bend, Indiana, Mayor Pete Buttigieg forms a presidential exploratory committee.
Jan. 25: Trump signs a bill to reopen the federal government after a 35-day shutdown, the longest in U.S. history.
Feb. 1: Sen. Cory Booker, D-N.J., jumps into the presidential race.
Feb. 5: Trump delivers his State of the Union address.
Feb. 10: During snowfall in Minnesota, Sen. Amy Klobuchar, D-Minn., announces her presidential bid.
Feb. 19: Sen. Bernie Sanders, I-Vt., enters the presidential race.
March 14: To great initial fanfare, former Rep. Beto O'Rourke, D-Texas, launches his presidential campaign.
March 24: Attorney General William Barr releases a partial summary of special counsel Robert Mueller's report on the Russia investigation and concludes that the evidence isn't sufficient to establish that Trump obstructed justice.
April 14: Buttigieg formally enters the 2020 race.
April 15: First-quarter fundraising reports are filed, revealing that Sanders ($18 million) and Harris ($12 million) raised the most money.
April 18: The full Mueller report is released, concluding that the Trump campaign welcomed Russia's interference in the 2016 election (although there was insufficient evidence that Trump or his aides conspired with the Russian government) and that Trump helped thwart the Russia investigation. "[W]hile this report does not conclude that the President committed a crime [of obstruction of justice], it also does not exonerate him."
April 25: Joe Biden enters the presidential race.
June 26-27: NBC News hosts the first Democratic debates, during which Harris confronts Biden on race in what would be a breakout moment of the campaign.
July 9: Tom Steyer joins the Democratic race.
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July 15: Second-quarter fundraising reports are filed, revealing that Buttigieg ($24.9 million), Biden ($22 million), Warren ($19.1 million) and Sanders ($18.2 million) raised the most money.
July 24: Mueller testifies before Congress, saying the Russia investigation was not a "witch hunt" or a "hoax."
July 25: Trump holds a phone call with Ukrainian President Volodymyr Zelenskiy, saying: "I would like you to do us a favor though. ... I would like you to find out what happened with this whole situation with Ukraine, they say Crowdstrike. ... The other thing, there's a lot of talk about Biden's son, that Biden stopped the prosecution, and a lot of people want to find out about that, so whatever you can do with the attorney general would be great."
July 30-31: CNN hosts the second round of Democratic debates in Detroit.
Sept. 6: Former Starbucks CEO Howard Schultz announces he won't run as an independent.
Sept. 12: ABC hosts the third Democratic debate in Houston.
Sept 24: House Democrats launch a formal impeachment inquiry into Trump's asking Ukraine's president to investigate Biden.
Oct. 1: Sanders suffers a heart attack while campaigning in Las Vegas.
Oct. 15: CNN and The New York Times host the fourth Democratic debate in Westerville, Ohio. Also, third-quarter fundraising reports are filed, revealing that Sanders, Warren and Buttigieg raised the most money.
Nov. 1: O'Rourke exits the presidential race.
Nov. 5: Democrats win the gubernatorial contest in Kentucky, while Republicans win in Mississippi; Democrats also take control of Virginia's Legislature.
Nov. 12: Mark Sanford, a former governor and House member representing South Carolina, ends his Republican primary challenge to Trump.
Nov. 16: Democratic Gov. John Bel Edwards of Louisiana wins re-election.
Nov. 20: Candidates participate in the fifth Democratic debate (hosted by MSNBC and The Washington Post) — on the same day Gordon Sondland, the U.S. ambassador to the European Union, testifies in the impeachment inquiry.
Nov. 24: Former New York Mayor Michael Bloomberg announces his presidential bid.
Dec. 3: Harris suspends her campaign.
Dec. 18: The Democratic-controlled House votes to impeach Trump, making him the third president in the country's history to be impeached.
Dec. 19: Candidates participate in the sixth Democratic debate (hosted by PBS and Politico) in Los Angeles as Warren and Klobuchar spar with Buttigieg.
Jan. 2, 2020: A U.S. military strike kills Iranian Maj. Gen. Qassem Soleimani.
Jan. 7: Iran retaliates by firing missiles at two Iraqi air bases housing U.S. forces.
Jan. 13: Booker exits the presidential race.
Jan. 14: Candidates participate in the seventh Democratic debate from Iowa (hosted by CNN and the Des Moines Register). Warren refuses to shake Sanders' hand after the event ends.
Jan. 15: The Democratic-controlled House submits articles of impeachment to the Senate.
Jan. 16: House Democratic impeachment managers read aloud the impeachment articles in the Senate, while Chief Justice John Roberts is sworn in to preside over the trial.
Jan. 22: The Democratic impeachment managers make their opening arguments in the Senate trial — after the chamber approves Majority Leader Mitch McConnell's organizing resolution on a party-line vote of 53-47.
Jan. 26: The New York Times reports on the unpublished manuscript for a book by former national security adviser John Bolton, which alleges that Trump told Bolton he wanted to continue withholding aid to Ukraine until it cooperated with investigations into Democrats — including the Bidens.
Jan. 31: A vote to call witnesses and documents in the impeachment trial fails, ushering in the final phase and an all-but-assured acquittal set for Wednesday.