Call it the Fox test.

When President Barack Obama decides to give a keynote interview to Fox News it is a sure sign that something is stirring.

For the past eight years, the president has mostly given a wide berth to the Right-wing cable channel that has been a stern critic of his administration and its policies - apart from two interviews, one of which took place during the Super Bowl.

Yet this weekend, Mr Obama will appear on Fox News Sunday with Chris Wallace in the middle of the rowdiest primary campaign in living memory in what can only be an eloquent indication of intent.

The last time he was interviewed by Wallace on the high profile Sunday show - one of American television’s political agenda-setters - was when the-then Illinois senator was running for president in 2008.