The US first lady and former first lady shared a warm embrace as they arrived on the platform.

"It doesn't get any better than being here with our most amazing first lady," opened Mrs Clinton.

Mrs Obama has previously made several emotional pleas endorsing Mrs Clinton and urging voters not to support her rival Donald Trump.

Mr Trump had steered clear of criticising the first lady until last weekend, when he commented that "all she wants to do is campaign". 

Responding to his jibe with Mrs Clinton sitting behind her, Mrs Obama said: "I know there are folks out there who've commented that it's unprecedented for a sitting first lady to be so actively engaged with a presidential campaign, and that may be true.

"What's also true is that this is a truly unprecedented election and that's why I'm out here."

She said Mr Trump's vision was based on "hopelessness and despair".

Mrs Clinton, on the other hand, was the most "qualified and prepared candidate for president  ... in our lifetime." 

The first lady referenced Mr Trump's attitude to foreigners and became visibly emotional as she talked about the contribution immigrants had made to the US, including her own ancestors. 

Having set aside the hard feelings of the 2008 primary between Mrs Clinton and her husband, Mrs Obama is seen as an important weapon for the Democratic candidate, who currently holds a 48.4% to 42.7% lead in the polls.