The race between Democratic presidential nominees Hillary Clinton and Bernie Sanders is much closer than most would have expected when campaigning began nearly a year ago. Other candidates have announced and dropped out, some potential heavy hitters within the party flirted with the idea of running and backed out, and the candidates have traveled many miles while traversing plenty of potholes along the way.
RELATED: How to watch the NBC News-YouTube Democratic debate
As Clinton, Sanders and former Maryland Governor Martin O'Malley prepare for Sunday night's NBC News-YouTube debate, here's a look back at how the Democratic race for president arrived at this juncture:
Hillary Clinton's Emails
Before Clinton — or any Democratic candidate — officially announced their candidacy for the nomination, controversy of Clinton's use of a private server when she was secretary of state dominated the discussion for the Democratic primary. It's an issue that hasn't gone away either as the State Department has released several thousand each month of the more than 50,0000 emails to be released. The FBI is also investigating and the Department of Justice has been asked to determine if the law was broken.
Elizabeth Warren. Will She or Won't She?
While it became increasingly evident that Clinton was going to mount a run, the progressive wing of the Democratic party actively urged Massachusetts Sen. Elizabeth Warren to mount a run. Activists hoped Warren would push Democratic proposals to the left, especially on economic issues of Wall Street regulation, entitlements and affordable higher education.
Clinton's Soft Open
Clinton officially entered the race on April 12 with a two-minute long video and a tweet. She doesn't appear in the video until the very end, when she says, "Americans have fought their way back from tough economic times, but the deck is still stacked in favor of those at the top," she said in a video posted on HillaryClinton.com. "Everyday Americans need a champion, and I want to be that champion."
Clinton's 'Scooby Van' And a Stop at Chipotle
The day after her video posted, Clinton hit the campaign trail. It was meant to be a laid back, easy-going Clinton embarking on a leisurely drive to Iowa, prompting her campaign to dub the vehicle she drove in "the Scooby Van" after the stoner-inflicted cartoon Scooby Doo. The trip quickly turned into a media frenzy with the press receiving little to no access to the Clinton leading to an over-reported stop to Chipotle and a media rampage chasing the Scooby Van through a parking lot.
Bernie Who?
Vermont Sen. Bernie Sanders, a little-known, long-time independent member of Congress who calls himself a socialist, decided to jump in the race. He announced in an interview with the Associated Press and simultaneously held a news conference outside the U.S. Capitol. He was polling below 5 percent while Clinton was polling above 60 percent at the time.
"People should not underestimate me," Sanders said at the time to the AP. "I've run outside of the two-party system, defeating Democrats and Republicans, taking on big-money candidates and, you know, I think the message that has resonated in Vermont is a message that can resonate all over this country."
Martin O'Malley Is In, Too
Former Maryland Gov. Martin O'Malley announced his bid in Baltimore. He launched his campaign positioning himself to the left of Clinton. His polling numbers neared 1 percent and even before he jumped in, his record as mayor of Baltimore came under the microscope after the death of Freddie Gray in police custody.
Sanders' is Shocked
Less than a month after his announcement, Sanders held a rally in Minneapolis. His senior adviser Tad Devine tells the story like this: When we drove up to the rally site, there were crowds of people and Sanders asked what is going on. Devine told him they are here to see him. Sanders couldn't believe it. He said he was "shocked."
More than 3,000 people showed up to the rally, far surpassing anyone's expectations, but it was a glimpse into the amount of support Sanders was amassing. He was also fulfilling the liberals' need for a more progressive candidate than Clinton.
Clinton's Unofficial Official Announcement
Clinton spent the first couple of months of her candidacy on a 'listening' tour. Finally in June, just two weeks after Sanders began to attract thousands to his events, Clinton held her first major rally. At Roosevelt Island, a land mass between Manhattan and Queens, Clinton said she wanted to be "in a place with absolutely no ceilings," referencing her bid to be the first female president.
Sanders On the Rise
Sanders' momentum continued through the summer. In just two months, Sanders support climbed 15 points, receiving an average of 21 percent support. Clinton's support, meanwhile, fell slightly. Her average approval hovered around 55 percent at the beginning of August. (Note that many of those polls included Biden who was contemplating a run.)
Sanders Shut Down By BLM
Then in early August Sanders was met with an obstacle that publicly posed a challenge for his campaign: his support among minorities and his commitment to them was questioned.
At a rally in Seattle, a Black Lives Matter protester climbed on stage in protest. Marissa Johnson was booed by the crowd and Sanders stood in silence. The event never restarted. After the campaign stop Sanders said he was “disappointed” that his rally was disrupted.
Joe Says No
After months and months of wavering, Vice President Joe Biden decides against a presidential run. He gave a passionate and emotional announcement in the Rose Garden with President Barack Obama and his wife Jill by his side. He said he was still deep in mourning for the loss of his son, Beau. "I believe we're out of time, the time necessary to mount a winning campaign for the nomination," Biden said.
The Others
Two other Democrats entered the race but did little campaigning. One-term Virginia Sen. Jim Webb's most covered component of the race was when he admitted killing people during the first Democratic debate. Former Rhode Island Sen. Lincoln Chaffee, who had also been a Republican and an independent, promoted the metric system at his presidential announcement and said he voted for a repeal of Glass-Steagall because it was his first vote in the Senate, implying that he didn't know what he was doing. Both have already dropped out.
'Damn Emails'
In what could be one of the most memorable moments of the Democratic primary, Sanders minimized the importance of Clinton's email controversy. "The American people are sick and tired of hearing about your damn emails!" Sanders exclaimed with firm agreement from Clinton during a Democratic debate.
O'Malley Serenades. Katy Perry Does it Better
At the popular fundraiser and pep rally for the Democratic base in Iowa, the Jefferson Jackson dinner, the candidates pulled out all the stops. Clinton brought Katy Perry. O'Malley serenaded with his guitar.
Sanders' Data Breach
A political bomb exploded just days before the fourth Democratic debate in mid December when members of Sanders' staff accessed Clinton's voter data file. The scandal led the firing of a Sanders staffer and the suspension of others. Sanders apologized to Clinton but the Democratic National Committee locked the Sanders campaign from its own voter data, resulting in the Sanders campaign to sue. The issue died quickly but revealed the animosity between the Democratic Party and the Sanders camp.
Enter Bill
Just one month from the Iowa caucuses, political heavyweight and former President Bill Clinton joined the campaign for Clinton. His entrance came as polls continued to tighten between Sanders and Clinton in the early states of Iowa and New Hampshire and the candidates battled over health care and guns.
This article first appeared on NBCNews.com.