Republican presidential nominee Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE is slamming President Obama for campaigning for Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE in Philadelphia, saying he should focus on his job and stay off the campaign trail.

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"Why isn't President Obama working instead of campaigning for Hillary Clinton?" Trump tweeted late Tuesday evening.

Why isn't President Obama working instead of campaigning for Hillary Clinton? — Donald J. Trump (@realDonaldTrump) September 14, 2016

Russia took Crimea during the so-called Obama years. Who wouldn't know this and why does Obama get a free pass? — Donald J. Trump (@realDonaldTrump) September 14, 2016

Obama earlier Tuesday criticized Trump's kind words for Russian President Vladimir Putin. The president also vigorously defended Clinton after a week during which the Democratic nominee said half of Trump's supporters fit into what she calls a "basket of deplorables."

She also had her health questioned after video surfaced of her being helped into a van following a 9/11 memorial in New York City on Sunday.

“We cannot afford, suddenly, to treat this like a reality show — we can’t afford to act as if there’s some equivalence here,” Obama said.

“What sets Hillary apart is that through it all, she just keeps on going, and she doesn’t stop caring, and she doesn’t stop trying, and she never stops fighting for us — even if we haven’t always appreciated it."