Presumptive Democratic presidential nominee Joe Biden tapped California Senator Kamala Harris on Tuesday to be his running mate ahead of the US election in November. 

Biden said Harris, who was widely seen as the front-runner in the former vice president's lengthy selection process, is "a fearless fighter for the little guy, and one of the country’s finest public servants."

"Back when Kamala was Attorney General, she worked closely with Beau. I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse," he said on Twitter, referring to his late son Beau. "I was proud then, and I'm proud now to have her as my partner in this campaign."

Harris now becomes the first Black and Asian-American woman in US history to be tapped as a vice presidential candidate and is the third woman overall to receive the role. Harris is biracial and is the daughter of a Black father and Indian-American mother. 

Biden "can unify the American people because he's spent his life fighting for us. And as president, he'll build an America that lives up to our ideals," she said on Twitter. "I'm honored to join him as our party's nominee for Vice President, and do what it takes to make him our Commander-in-Chief."

Biden and Harris are expected to deliver their first remarks under a joint presidential ticket on Wednesday.

President Donald Trump quickly responded to the news with a campaign video he released on Twitter, apparently prepared in advance, accusing the duo of jointly embracing "the radical left."

"Slow Joe and Phony Kamala. Perfect together. Wrong for America," the video says. 

Harris is joining the race with Biden holding a commanding lead over Trump. A poll released earlier Tuesday from Quinnipiac University shows the former vice president 10 points ahead of Trump -- 51% to 41%.

The US is reeling from its handling of the coronavirus pandemic, with the highest number of confirmed cases and fatalities in the world. The American public has widely lambasted Trump for his handling of the outbreak, a major factor in his dip in opinion polling. 

Biden selected Harris from a particularly crowded field of female contenders, including Florida Congresswoman Val Demings, Atlanta Mayor Keisha Lance Bottoms and former National Security Advisor Susan Rice.

Rice congratulated Harris, calling her in a statement a "tenacious and trailblazing leader who will make a great partner on the campaign trail."

Former President Barack Obama also lauded the decision, saying Biden "nailed" it. 

"By choosing Senator Kamala Harris as America’s next vice president, he’s underscored his own judgment and character. Reality shows us that these attributes are not optional in a president. They’re requirements of the job," Obama said. "And now Joe has an ideal partner to help him tackle the very real challenges America faces right now and in the years ahead."