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Matt Rourke, Associated Press
Democratic presidential candidate, Sen. Bernie Sanders, I-Vt. speaks during a campaign stop, Thursday, April 7, 2016, at the Pennsylvania AFL-CIO Convention in Philadelphia.

NEW YORK — The race for the Democratic nomination took a decidedly negative turn, with Hillary Clinton and Bernie Sanders exchanging a series of barbs over each other's qualifications for the presidency.

In overlapping press conferences, the two Democratic primary rivals addressed the ping-ponging accusations, with Sanders vowing to fight back against Clinton's accusations.

"This is not the type of politics that I wanna get in," he told journalists in Philadelphia. "But we'll get used to it fast. I'm not gonna get beat up. I'm not gonna get lied about."

Clinton, campaigning in New York City, sought to shift attention back to her Republican opponents, saying: "I will take Bernie Sanders over Donald Trump or Ted Cruz any time, so let's keep our eye over what's at stake in this election."

The testy exchanges between the two candidates on Wednesday marked a notable tonal shift for a primary contest that's remained largely civil since voting began in February. Now, as the race moves toward the make-or-break New York primary, the stakes have grown high for both camps, with Sanders' recent string of victories complicating Clinton's efforts to march headstrong toward the general election.

Sanders told a crowd of more than 10,000 people in Philadelphia on Wednesday that Clinton has been saying that he's "not qualified to be president."

"I don't believe that she is qualified if she is, through her super PAC, taking tens of millions of dollars in special-interest funds," he said.

Clinton launched a flurry of attacks against Sanders earlier that day, questioning his truthfulness and policy expertise though she stopped short of saying he was unqualified for the job.

In a discussion of Sanders interview with the editorial board of the New York Daily News, Clinton was asked if "Bernie Sanders is qualified and ready to be president of the United States."

She responded, "Well, I think he hadn't done his homework and he'd been talking for more than a year about doing things that he obviously hadn't really studied or understood, and that does raise a lot of questions."

Despite her sizable delegate lead, a loss in the April 19 contest would be a major political blow for Clinton that would highlight her weaknesses within her own party, particularly with younger voters who've powered Sanders primary bid.

A former New York senator, she's been touting her work in Congress for the state, highlighting her economic record in visits to struggling upstate cities.

On Thursday, she took a quick jaunt on the New York City subway, riding the train one stop in the Bronx. Walking along East 170th Street afterward, she stopped to shake hands and greet a baby in a stroller. "I need your vote!" she told one man before dropping into Munch Time, a cafe near Townsend Avenue.

The photo op was aimed at Sanders, who told the New York Daily News in an interview earlier this week that New Yorkers still used tokens to pay for the train. The system switched over to pre-paid cards in 2003.

"I think it was my first term when we changed from tokens to MetroCards," Clinton said laughing, as she talked to reporters before boarding the train.

A Brooklyn native, Sanders left New York for Vermont in 1968.

Still, he's cast himself as a native son of the state, viewing the contest as a springboard into primaries out West later in the summer and a pathway to closing his more than 250-delegate gap with Clinton.

The Vermont senator must win 68 percent of the remaining delegates and uncommitted super delegates if he hopes to clinch the Democratic nomination. That would require blow-out victories by Sanders in upcoming states big and small, including New York.

In a separate interview with Politico published on Wednesday, Clinton said she tries to explain things in a more "open and truthful way than my opponent."

Later, at a Philadelphia job training center, Clinton said people should know what she would do if she's elected president, "not just lots of arm-waving and hot rhetoric."

Ignoring their own barbs, Clinton aides pushed back on Sanders' attack, with spokesman Brian Fallon writing on Twitter: "Hillary Clinton did not say Bernie Sanders was 'not qualified.' But he has now — absurdly — said it about her. This is a new low."

Whack reported from Philadelphia. Associated Press writer Lisa Lerer contributed to this report.