Why being called Mr Leader by Mitt Romney may be a mark of respect

I’m no fan of Mitt Romney, but I’m bemused by the amusement today (including over at the BBC) after Mitt Romney referred to Ed Miliband as “Mr Leader”.

Except in American politics, the leaders of Majority and Minority groups in the Senate and Congress are habitually referred to (reverentially) as “Mr Leader”.

Such as this interview with Senate Majority leader Eric Cantor on Meet The Press:

Ed Miliband is – as an American politician would understand it – the “Minority Leader”. Mr Leader, therefore, is a term of respect, right?