Simple.   If you donate $100 to the CPC, $75 gets refunded to you in the form of a tax credit.   In other words 75% of your donation to the CPC is funded by the taxpayer.   The same for donations to the Liberals and to the NDP.

It has nothing to do with running deficits.    A tax dollar is a tax dollar regardless of whether a Govt is running a surplus or a deficit.     Govt's don't generate revenue.