All Iggy needs to say is:
"To even begin to discuss such a fantasy scenario is to admit defeat... To admit that we won't finish ahead of the Conservatives in this election. I am NOT about to do that."
"Now why don't you go ask Mr. Harper that very same question, if he finishes second - and judging by the direction of recent polls, he just might by election day."
The point is, either party could finish second. Would Harper consider a coalition if he finished second? Would he even discuss it now (no)?
In politics, like in sports, you NEVER speculate about finishing 2nd. It's bloody idiotic.
Iggy just needs to spell that out for the press. He can also add, "We will only do what is traditional and permissible in our Parliamentary Democracy... The people - the Parliament - are the ultimate determinants of what will transpire."