Former US vice-president Joe Biden entered the second debate of Democratic presidential candidates with a target on his back. California Senator Kamala Harris hit the bullseye.

Harris attacked Biden for working with Southern segregationists, and in doing so, revived her flagging campaign by showing she's tough and can command the stage, two attributes a nominee will need to challenge President Donald Trump in 2020.

"I do not believe you are a racist," Harris said looking directly at Biden. "And I agree with you when you commit yourself to the importance of finding common ground. But I also believe it was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on segregation of race in this country. And it was not only that, but you also worked with them to oppose busing."

Busing is transporting a child of one race to a school where another race is predominant, in an attempt to promote racial integration.

Biden's voting record - including his support for civil rights - is well-known, but Harris made the issue personal.

"There was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day," Harris said. "That little girl was me."

On the day after the debate, Biden gave a lengthy and vigorous defense of his civil rights record, saying that "30 seconds to 60 seconds on a campaign debate exchange can't do justice to a lifetime committed to civil rights".

"I never, never, never ever opposed voluntary busing," he said at a luncheon in Chicago.

Biden responded to Harris in the debate that he did not praise his fellow Democrats for their racial views, as Senator Corey Booker of New Jersey claimed, but worked with them as needed to get bills through the Senate.

"I felt Harris hit Biden with a punch he wasn't expecting," Jonathan Zogby, CEO of Zogby Analytics, which provides custom research and insight to leaders of businesses and communities, told China Daily. "Harris damaged Biden and helped herself. Harris was disingenuous, but she's trying to break away from the pack. When going up against Trump, the Democrats need someone who can get in the mud and sling it."

Zogby said Biden can recover, but he must be better prepared for the next debate.

Andrew Smith, a professor of political science at the University of New Hampshire and director of the school's survey center, said Harris's attack on Biden wasn't a surprise.

"That's politics," he said. "As Nixon said in 1960 when running as a Republican, you have to run to the right in the primaries and then to the center in the general election. That's a pivot all candidates have to make and positioning themselves is a test of their political skill."

After four hours of debate last week involving 20 candidates for the Democratic Party's presidential nomination, there are mixed reviews about how aspirants performed, but there is one clear winner: undocumented immigrants.

If Democrats recapture the White House in 2020, the candidates promised to provide healthcare coverage for those who cross the southern US border illegally. Polls show the proposal is not supported by a majority of Americans.

Savannah Gutherie, one of the moderators at both debates, said, "Raise your hand if your government (health) plan would provide coverage for undocumented immigrants." On the second debate on June 27, all candidates raised their hands.

But then president Barack Obama's healthcare plan, signed into law in 2010, excluded illegal immigrants. "Why extend coverage to a group Democrats previously excluded?" Gutherie asked.

Biden responded, "You cannot let people who are sick, no matter where there come from, no matter what their (immigration) status, go uncovered. You can't do that. It's just going to be taken care of, period. You have to. It's the humane thing to do."

Critics say such a policy would provide further incentive to cross the southern US border illegally. President Donald Trump tweeted, "All Democrats raised their hands for giving millions of illegal aliens unlimited healthcare. How about taking care of American citizens first? That's the end of the race!"

The exchange underscores that the Democratic Party has moved left since Biden's tenure as vice-president ended in 2017. The "progressive" tenor of the debates also calls into question Biden's appeal as a candidate, despite his large lead in early polls, and about the wisdom of the Democrats' tactics leading up to the 2020 campaign, analysts said.

"The Democrats' stance on immigration plays well among the coastal elites but won't play where it matters - the Midwest and battleground states," Zogby said. "Some voters will say, 'You've forgotten about us.' I think this issue plays to Trump's advantage."

While the Democrats offer new programs and promise free goodies to illegal immigrants, Trump will tout the strong economy, rising income and record low unemployment among blacks and Hispanics, analysts said.

"The Democrats are talking tax and spend," Zogby said. "Trump can say he's put money in people's pockets in a strong economy."

Biden leads the race for the Democratic Party's nomination. According to the Real Clear Politics, a Chicago-based political news and polling data aggregator, the average of major polls shows he is favored by 32 percent of primary voters, with Sanders second at 16.9 percent, Warren third at 12.1 percent and Harris fourth at 7 percent. Most of the other candidates poll at less than 1 percent.

Senator Elizabeth Warren of Massachusetts was understated during her appearance in the first debate, but made the points she wanted to make, and established herself as the alternative to Senator Bernie Sanders of Vermont. He's still the only socialist curmudgeon, but this time Sanders appears crankier and less appealing than he did in 2016 when he was the alternative to the unpopular Hillary Clinton.