Democratic presidential contender Joe Biden hit back at President Trump in a tweet Thursday, after Trump hurled insults at Biden and his son, Hunter, at a Minneapolis rally.

"I spent my night at the HRC forum talking about the fundamental respect every human being deserves. You spent yours showing how little respect for anyone else you have. America is so much stronger than your weakness," tweeted Biden, after appearing at a town hall event on LGBT rights.

Trump told cheering supporters earlier: "Hunter, you're a loser. Why did you get $1.5bn, Hunter? And your father was never considered smart. He was never considered a good senator. He was only a good vice president because he understood how to kiss Barack Obama's ass."

Democrats have launched an impeachment inquiry after Trump pressured Ukraine to investigate Biden and his son.

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Democratic presidential contender Joe Biden hit back at US President Donald Trump, who in a rally in Minneapolis Thursday night had launched a series of insult-laden attacks at him and his son, Hunter, saying the former vice president was "never considered smart."

After appearing at a CNN town hall event in Los Angeles to discuss LGBT rights, Biden tweeted: "I spent my night at the HRC [Human Rights Campaign] forum talking about the fundamental respect every human being deserves.

"You spent yours showing how little respect for anyone else you have. America is so much stronger than your weakness."

Read more: Pillows, Pelosi, and 'Mr. President': 10 of the wildest moments from Trump's first rally since the impeachment inquiry

President Donald Trump addresses a campaign rally Thursday, Oct. 10, 2019, in Minneapolis. AP Photo/Jim Mone

Earlier, Trump had launched his most scathing and coarse attacks to date on Biden and his son.

Referring to claims that Biden secured a billion dollar loan for a fund he ran while visiting China with his father, Trump attacked the Bidens as supporters cheered.

Read more: The life of Hunter Biden: How Joe Biden's middle child landed in Trump's Ukraine scandal

"Hunter, you're a loser. Why did you get $1.5 billion, Hunter? And your father was never considered smart. He was never considered a good senator. He was only a good vice president because he understood how to kiss Barack Obama's ass."

Hunter Biden has denied profiting from the 2013 investment from the Bank of China investment in the BHR fund, where her served on the board of directors.

The president currently faces one of the most serious crises of his presidency, after Democrats launched an impeachment inquiry into a whistleblower's claim Trump abused his office in pressuring Ukraine to investigate the Bidens.