BRONX, New York -- With accusations of lying, hustling for money and failed leadership, the race for the Democratic nomination took a decidedly negative turn, with Hillary Clinton and Bernie Sanders exchanging a series of barbs over qualifications for the presidency.The testy exchanges underscored the heightened stakes for both sides as the race turns to New York, where Sanders hopes to turn his recent winning streak into concrete momentum toward the nomination. Clinton, meanwhile, is looking to the April 19 contest to take command of a primary race that many in her campaign worry will only amplify her weaknesses heading into the general election.Sanders path to the nomination remains narrow: 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.Lagging in delegates and under fire from a frustrated Clinton, Sanders is shifting away from his pledge to avoid negative attacks and stinging her with direct accusations."I will not leave here this morning and go to a Wall Street fundraiser," he told union members at an AFL-CIO conference in Philadelphia on Thursday. "I will not be hustling money from the wealthy and the powerful."The comment was a direct dig at Clinton, who was headed to Ohio and Colorado for fundraising after a campaign stop in New York City."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 sought to shift attention back to her Republican opponents, telling reporters Thursday: "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."Clinton sought to shift attention back to her Republican opponents, telling journalists on Thursday: "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 race for the Democratic nomination has remained relatively civil as compared to a chaotic and crowded Republican race colored by flagrant attacks.Clinton has spent much of the past few weeks focused on Trump and Cruz, hoping to rally her party behind her by warning of regression to President Barack Obama's achievement should a Republican win the election. But after Sanders logged a big win in Wisconsin on Tuesday night - his sixth victory in the last seven contests - she's been forced to pivot back to her primary opponent.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 two stops 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 MetroCards in 2003.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 along the Eastern seaboard in April and out West later in the spring and a pathway to closing his more than 250-delegate gap with Clinton.Clinton unleashed a flurry of attacks against Sanders on Wednesday, 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."Sanders jumped on the remark, telling a crowd of more than 10,000 people in Philadelphia on Wednesday night 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.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."