WASHINGTON, June 2 (Xinhua) -- The U.S. Democratic Party's sole remaining presidential candidate Joe Biden continued to march toward official nomination Tuesday, as presidential primaries were held in seven states and the District of Columbia.

As of Tuesday evening, the former vice president has won primaries in Indiana, Rhode Island, Pennsylvania, Maryland, Montana, New Mexico and South Dakota. Voters are also casting ballots in the District of Columbia.

Biden, already the presumptive nominee, is edging closer to winning 1,991 delegates, a threshold for nomination. He will have to win 89 percent of the 479 delegates up for grabs Tuesday to formally clinch the nomination.

Though Bernie Sanders, the last candidate to drop out of the Democratic race, has claimed to endorse Biden, he remains on the ballot in a bid to influence the party's platform, which might slow down Biden's pace in becoming the nominee.

Tuesday's primaries were special because they were held not only amid the coronavirus pandemic so that mail-in voting, instead of in-person, is encouraged; but also during nationwide protests against police brutality and racial injustice following the death of George Floyd, an unarmed black man, in police custody.

In Washington D.C. and Pennsylvania, voters were still seen casting their ballots at polling stations despite the imposition of curfews.

Earlier in the day, Biden said in a speech in Philadelphia that the "I can't breathe" cry from Floyd during the final moment of his life is a "wake-up call" for all Americans, and that he, not President Donald Trump, can provide the much-needed leadership that serves to unite the nation.

"I won't traffic in fear and division. I won't fan the flames of hate. I will seek to heal the racial wounds that have long plagued this country - not use them for political gain," the 77-year-old said.

"I'll do my job and take responsibility. I won't blame others. I'll never forget that the job isn't about me," he stressed.