The Federal political contribution tax credit is on Line 41000. Line 40900 would just show the total amount that you contributed.
The political contribution credit is non-refundable (except the Ontario credit). This means you only get it if you need it to reduce your taxes owing. So, if you don’t have taxes owning, you won’t get the credit. If you only have some taxes owing, then you'll only get enough of the credit to reduce your taxes to zero.
Since Political Contributions 41000 are listed in the section which is "Refund or Balance Owing" why wouldn't such a contribution, which would be a credit, offsetting any taxes added in that section e.g at 42800, and then net out at 43500?
It defies logic that anyone with a refund should be unable to receive such a credit. Since a huge number of taxpayers do claim refunds this will thwart fundraising efforts by political parties.
As per the CRA: Enter your total federal political contributions on line 40900 of your return.
If your total federal political contributions from line 40900 of your return were $1,275 or more, enter $650 on line 41000 of your return.
If not, use the amount from line 40900 to calculate your federal political contribution tax credit using the chart for line 41000 on your Federal Worksheet. Enter the result on line 41000 of your return.