A couple of assumptions that you did not state. You assume that your favored candidate's budget contains truly optimal uses of charitable dollars. You need a step down function unless your preferred charity is funding government programs.

You assume that the opposition candidate's spending is valueless. Otherwise you need to consider the relative merits.

You assume that there is no portion of the opposition budget that is preferable. If you believe that each candidate has some portions right, you need to be subtracting this spending from the value of your contribution.

You assume that the proposed budget will be implemented. Given the track record of campaign promises, this is an iffy assumption. As this probability is necessarily less than 100% it should reduce the value of your contribution.

These assumptions are the mind killing biases of politics.