'He pumps himself up by putting other people down': Obama blasts 'insecure' Trump and his 'disturbing' behavior hours before Republican candidate's showdown with Hillary Clinton 

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President Obama blasted Donald Trump hours before the second presidential debate on Sunday, labeling the Republican candidate 'insecure' and 'disturbing'.

He did not even acknowledge the businessman by name as he chipped away at his 'insulting' campaign at a fundraiser for Democratic representative Tammy Duckworth, in Chicago, Illinois. 

'It tells you that he's insecure enough that he pumps himself up by putting other people down,' Obama quipped, listing what he considered to be the candidate's most offensive moments to date.

'One of the most disturbing things about this election is just the unbelievable rhetoric coming from the top of the Republican ticket. 

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President Obama delivered a scathing critique of Donald Trump on Sunday, labeling the Republican presidential candidate 'insecure'

'I don't need to repeat it. There are children in the room. 

'But, demeaning women, degrading women, but also minorities, immigrants, people of other faiths. Mocking the disabled. Insulting our troops. Insulting our veterans. 

'That tells you a couple of things,' he said to applause, NPR reported. 

It came hours before the billionaire's second showdown with Democratic candidate Hillary Clinton. 

The pair will face-off in St Louis, Missouri, at 9pm in what is expected to be their scrappiest meeting yet. 

At the top of the agenda is Trump's 2005 comments that he could 'grab' women 'by the p****' and do 'anything' to them because he was a star. 

The remarks, caught on a hot-mic during a guest appearance on Access Hollywood, were released on Friday and sent the GOP into a tail-spin. 

The President chastised Trump for 'degrading, demeaning and insulting' women as he spoke at a senatorial campaign fundraiser for Democratic representative Tammy Duckworth in Chicago, Illinois

Trump (seen above leaving Trump Tower in New York for St Louis, Missouri, on Sunday) has refused to drop out of the race despite the withdrawal of key GOP figures' endorsements 

Dozens have withdrawn their support of the candidate who, despite acknowledging the comments' impropriety, has refused to drop out of the race for the White House.

Among those to have publicly condemned his words are former Secretary of State Condoleezza Rice and Trump's own running mate Mike Pence. 

Vice President Joe Biden explosively likened Trump's boasts about advancing on married women to sexual assault while Speaker of the House Paul Ryan said he was left 'sickened' by the comments. 

Melanie Trump joined them in their rebuking of her husband but pleaded with voters to forgive him as she had. 

On Saturday night, more incriminating tapes of the candidate emerged. 

In a string of conversations taped over 17 years, he jeered with Howard Stern over women's body types, their ages and how owning a collection of beauty pageants brought him special privileges to flirt with contestants. 

The defiant candidate (above, posing for a photograph with campaign manager Kellyanne Conway) acknowledged his 2005 comments about women were 'wrong' but vowed to continue in his bid for the White House 

President Obama said the comments were 'disturbing', refusing to even use the candidate's name as he spoke on Sunday

Clinton touched down in St Louis, Missouri, on Sunday ahead of the debate. She will be the first to speak after winning a coin toss 

The pair will square off in what is set to be the scrappiest of their meetings at St Louis's Washington University (above) on Sunday at 9pm

Trump suggested he would bring up rape allegations made against his opponent's husband Bill Clinton (pictured above) on Sunday 

In perhaps the most shocking of the recordings, Trump agreed with Stern's description of his daughter, Ivanka, as a 'piece of ass' and said his wife became a 'blimp' and a 'monster' when she was pregnant with their only son. 

The defiant Donald brushed off the scandal on Saturday, emerging from Trump Tower on 5th Avenue, New York City, to the cheers of undeterred supporters. 

On Sunday he continued his determined dismissal of the scandal as he prepared for the debate, hinting that he may use the platform to resurface rape claims against his opponent's husband.

Trump re-tweeted the comments of Juanita Broaddrick who claims Bill Clinton raped her in 1978. 

He also slammed defective members of his own party who have retreated from supporting him since Friday.  

'Tremendous support (except for some Republican "leadership"). Thank you,' he said. 

The first question of Sunday's debate will be answered by Clinton who will no doubt use the opportunity to deliver her first brutal blow of the night.  

Trump has lost the support of key endorsers since a 2005 recording of him boasting that he could 'grab' women 'by the p****' emerged on Friday (above) 

Later, recordings of Trump discussing sex, women's bodies and how he could flirt with Miss Universe contestants, with radio personality Howard Stern (above with their wives in 2005) resurfaced