The elections took place within all fifty U.S. states, the District of Columbia, five U.S. territories, and Democrats Abroad and occurred between February 1 and June 14, 2016.

Six major candidates entered the race starting April 12, 2015, when former Secretary of State and New York Senator Hillary Clinton formally announced her second bid for the presidency.

Clinton then secured numerous important wins in each of the nine most populous states including California, New York, Florida, and Texas, while Sanders scored various victories in between.

[16][17][18][19][20] This was evidenced by alleged bias in the scheduling and conduct of the debates,[c] as well as controversial DNC–Clinton agreements regarding financial arrangements and control over policy and hiring decisions.

[52][53] This polling information prompted numerous political pundits and observers to anticipate that Clinton would mount a second presidential bid in 2016, entering the race as the early front-runner for the Democratic nomination.

[62][63] However, on October 21, 2015, speaking from a podium in the Rose Garden with his wife and President Obama by his side, Biden announced his decision not to enter the race, as he was still dealing with the loss of his son, Beau, who died weeks earlier at the age of 47.

[70] He emerged as the biggest rival to Hillary Clinton in the Democratic primaries, backed by a strong grassroots campaign and a social media following.

Senator who had once served as the U.S. Secretary of the Navy during the Reagan administration, announced the formation of an exploratory committee in preparation for a possible run for the Democratic presidential nomination.

[74] Martin O'Malley, former Governor of Maryland as well as a former Mayor of Baltimore, made formal steps toward a campaign for the party's nomination in January 2015 with the hiring and retaining of personnel who had served the previous year as political operatives in Iowa – the first presidential nominating state in the primary elections cycle – as staff for his political action committee (PAC).

O'Malley had started the "O’ Say Can You See" PAC in 2012 which had, prior to 2015, functioned primarily as fundraising vehicles for various Democratic candidates, as well as for two 2014 ballot measures in Maryland.

[76] In August 2015, Lawrence Lessig unexpectedly announced his intention to enter the race, promising to run if his exploratory committee raised $1 million by Labor Day.

[80][81] Despite being heavily favored in polls issued weeks earlier, Clinton was only able to defeat Sanders in the first-in-the-nation Iowa Caucus by the closest margin in the history of the contest: 49.8% to 49.6%.

[83] The victory, which was projected to award her 23 pledged national convention delegates, two more than Sanders, made Clinton the first woman to win the Caucus and marked a clear difference from 2008, where she finished in third place behind Obama and John Edwards.

[84][85][86][87] Martin O'Malley suspended[b] his campaign after a disappointing third-place finish with only 0.5% of the state delegate equivalents awarded, leaving Clinton and Sanders the only two major candidates in the race.

[89][90][91] Hillary Clinton's loss in New Hampshire was a regression from 2008, when she defeated Obama, Edwards, and a handful of other candidates including Joe Biden, with 39% of the popular vote.

[93][94] For her part, Clinton, who had won the state eight years prior in the 2008 Nevada Democratic caucuses, hoped that a victory would allay concerns about a possible repetition of 2008 when she ultimately lost to Obama despite entering the primary season as the favorite for the nomination.

Although the results overall were unfavorable for Sanders, his four wins and narrow loss allowed him to remain in the race in anticipation of more favorable territory in New England, the Great Plains, Mountain States and the Pacific Northwest.

[100] Sanders found more hospitable ground on the weekend of March 5, 2016, winning caucuses in Kansas, Maine and Nebraska by significant margins.

Clinton answered with an even larger win in Louisiana's primary, limiting Sanders' net gain for the weekend to only four delegates.

[110] Missouri state law allowed for a possible recount had any of the candidates requested it; however, Sanders forwent the opportunity on the basis that it would not significantly affect the delegate allocation.

[114] Despite continued efforts by Sanders to close the gap in Arizona after his surprise win in Michigan, Clinton won the primary with 56.3% of the vote.

[131] On June 6, both the Associated Press and NBC News reported that Clinton had sufficient support from pledged and unpledged delegates to become the presumptive Democratic nominee.

[135] Clinton finally declared victory on the evening of June 7, as the results ensured that she had won a majority of the pledged delegates and the popular vote.

[135] Sanders stated he would continue to run for the Democratic Party's nomination in the final primary in the District of Columbia on June 14,[136] which Clinton won.

[139] On July 22, 2016, WikiLeaks released online tens of thousands of messages leaked from the e-mail accounts of seven key DNC staff.

Others showed a few staffers had expressed personal preferences that Clinton should become the nominee, suggesting that the party's leadership had worked to undermine Bernie Sanders' presidential campaign.

[140] The furor raised over this matter escalated to Wasserman Schultz's resignation ahead of the convention,[141] and that of Marshals, Dacey, and Communications Director Luis Miranda afterwards.

[142] Following Wasserman Schultz's resignation, then-DNC Vice Chair Donna Brazile took over as interim DNC chairwoman for the convention and remained so until February 2017.

"[150] After the general election, the U.S. intelligence community and the Special Counsel investigation assessed that the leaks were part of a larger interference campaign by the Russian government to cause political instability in the United States and to damage the Hillary Clinton campaign by bolstering the candidacies of Donald Trump, Bernie Sanders, and Jill Stein.

[165] For the U.S. territories of Puerto Rico, American Samoa, Guam, the U.S. Virgin Islands and for Democrats Abroad, fixed numbers of pledged delegates are allocated.