Sanders, in Philadelphia, did not back down from his assertion that Clinton's Wall Street ties, among other things, disqualify her for the office.

“She has attacked me for being unqualified, and if I am going to be attacked for being unqualified, I will respond in kind,” Sanders told reporters. "I'm not going to get lied about."

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He added: “If Secretary Clinton thinks that I just come from the small state of Vermont, that we’re not used to this, well, we’ll get used to it fast. I’m not going to get beaten up; I’m not going to get lied about. We will fight back.”

The dueling news conferences were part of a fierce back and forth that began Wednesday. Sanders, in a speech, accused Clinton of saying he was not qualified. She had not said so outright but had spent the day Wednesday raising questions about his ideas, plans and preparation for the job.

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Clinton didn't back down Thursday, either.

"I'm going to keep talking about the issues," Clinton said, "and I'm going to keep drawing contrasts, because that’s what elections are about."

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"I think it is important to tell people what you're going to do for them and how you can get it done, how you can produce results that will make a positive difference in people's lives," she added.

That was a reference to her frequent claim that many of Sanders's liberal proposals aren't realistic.

"Don't make promises you can't keep," she said.

Her unusual ride on the 4 train, covered live by her traveling press, followed an awkward New York Daily News interview in which Sanders appeared not to know that the vast New York subway system no longer uses metal tokens.

At her news conference Clinton noted, laughing, that the system switched to farecards years ago -- during her first term as a New York senator.

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Clinton’s campaign charged earlier Thursday that Sanders “crossed a line” by asserting that Clinton isn't qualified for the White House.

“This is a ridiculous and irresponsible attack for someone to make — not just against the person who is almost certainly going to be the nominee of their party this November, but against someone who is one of the most qualified people to run for the presidency in the HISTORY OF THE UNITED STATES,” Clinton spokeswoman Christina Reynolds wrote in a fundraising email to supporters early Thursday.

Clinton planned to spend much of the rest of the day Thursday raising money. She is trying to fend off a strong challenge from Sanders in her adopted home state, as the formerly cordial Democratic race becomes increasingly nasty.

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Sanders addressed the Pennsylvania AFL-CIO in Philadelphia on Thursday morning before heading to New York.

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He told the group that he considers labor unions to be family and then took a dig at Clinton: “I will not leave here after this morning and go to a Wall Street fundraiser.”

Clinton had addressed the same body Wednesday, when she questioned whether Sanders’s proposals are realistic or sound. For some important issues, she asserted, Sanders has no plan at all.

“He hasn’t done his homework,” Clinton said earlier Wednesday on MSNBC, adding that it was up to voters to weigh his qualifications. She continued that theme throughout the day, which prompted Sanders to comment that Clinton was not qualified, in part because of her approval of the Iraq War.

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Speaking at a rally here Wednesday, Sanders ticked off a string of reasons he believes Clinton is unqualified to be president, including her support of trade agreements, her vote for the Iraq War and her acceptance of “special interest money.”

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“This is a new low,” Clinton spokesman Brian Fallon said on Twitter.

The fundraising email asked supporters to "chip in" as little as $1 to help fight back in the war of words.

Sanders repeated the critique of Clinton's qualifications at a Thursday morning news conference.

Highlights from Bernie Sanders’s campaign, in pictures share Share View Photos View Photos Next Image WASHINGTON, DC- JUNE 14: Presidential candidate, Bernie Sanders arrives at the Capital Hilton to meet with Hillary Clinton on Tuesday June 14, 2016. (Matt McClain/ The Washington Post)

“If you want to question my qualifications, let me suggest this, maybe the American people might question your qualifications, Madame Secretary,” the senator from Vermont said. He also took aim at Clinton’s past support for “disastrous” trade deals.

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Speaking to reporters at a Philadelphia hotel Thursday morning, Sanders said his broadside was inspired by a Washington Post headline that said Clinton had questioned his qualifications, as well as by a CNN report that the Clinton campaign is seeking to “disqualify” him in advance of the key April 19 primary in New York.

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“That’s what was thrown at me,” Sanders said, adding that Clinton is mistaken if she believes a senator from a small state is not capable of playing rough-and-tumble New York politics.