Why do you think Mulcair is so similar to Harper?
Let's see:
Strong focus on balancing the budget rather than stimulus and program funding
Stay the course on child benefit
Stay the course on personal income taxes
Support for international trade agreements
Pro-Israel (or more strongly so than the NDP has tended to be in the past).
And, perhaps most damningly, his own left wing has released a manifesto that runs counter to much of what he is pushing as the NDP platform (the Leap Manifesto), showing just how much he has moved the party away from its former position.
To be fair, I'm not sure he's so much similar to Harper as to a centrist Liberal like Paul Martin, but he's certainly not following in the footsteps of NDP leaders past like Ed Broadbent and Audrey McLaughlin. Even Layton, who did move the party a bit more to the centre, was still to the left of where Mulcair seems to be going. The fact that he started out as a Liberal (albeit a Quebec Liberal, who tend to sit on the left of the party) likely says a lot about where he is going.
In the end, I don't expect an NDP government to become a centre-right rival for the Tories, but under Mulcair they do seem to be staking a more centre to centre-left position akin to where the Liberals have sometimes gone in the past - basically fiscally conservative but socially liberal. A rather different beast from the socialist party of Lewis, Broadbent, and their successors, which is already raising some alarms from the party base.
If the NDP does not finish at least second in this election, expect to see him go down and the party shift back left. Just witness what's happening with Labour in the UK. Blair and Brown shifted them strong to the centre (even to the right on some issues) but now that they're stuck well away from power, they've put a traditional socialist Labour leader in and are starting to move away from the positions taken by Blair and Brown.