As Americans commemorate Martin Luther King Jr.'s contributions to the nation, Democratic presidential hopefuls are fanning out across the country to honor the civil rights leader.

Martin Luther King addresses crowds during the March On Washington at the Lincoln Memorial, Washington DC, where he gave his 'I Have A Dream' speech in 1963. Central Press | Getty Images

Sen. Kamala Harris, D-Calif., used the holiday to launch a presidential campaign that, if successful, would make her the first woman and the second black candidate to become president. Meanwhile, an annual rally to observe King's birthday held in the capital of South Carolina, a critical early-voting state in the Democratic primary, will feature two senators expected to seek the White House in 2020, Cory Booker of New Jersey and Bernie Sanders of Vermont. Former Vice President Joe Biden, who's weighing his own presidential bid, is set to speak at a King holiday event in Washington alongside former New York mayor and possible 2020 rival Michael Bloomberg. Two candidates who have already opened exploratory committees — Sens. Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York — will also appear at King-centered events. While the Democratic field for 2020 is only beginning to take shape, the year that would have marked King's 90th birthday gives the party's prominent members a valuable opportunity to address race and, potentially, draw a contrast between their own views and those of President Donald Trump, whose approach to questions of racial justice has sparked criticism from multiple minority groups since he took office. How Democratic contenders, both those officially in the race and those still mulling campaigns, celebrated the King holiday:

Kamala Harris

Harris, a first-term senator and former California attorney general known for her rigorous questioning of Trump's nominees, opened the holiday by declaring her bid on ABC's "Good Morning America." She abandoned the formality of launching an exploratory committee, instead going all in on a presidential campaign.

Sen. Kamala Harris (D-CA) speaks to Amos Jackson III, Executive President of the Howard University Student Association, and Mara Peoples, Executive Vice President, after announcing her candidacy for President of the United States, at Howard University, her alma mater, on January 21, 2019 in Washington, DC. Al Drago | Getty Images