How to calculate a % in a dataframe given certain conditions in R, use ifelse statements?

I'm fairly new to R and am trying to do a little data analysis of congressional elections in the US (for fun as well as a project to practice R skills)

SO I have a CSV of all federal congressional elections since 1976. What I am trying to do is get an average of Dem/Rep margin across the US grouped by election year. So for example I'll have: in 1976 Dems won the house by X%, in 1978 lost by Y%, etc.

This is fairly trivial to do in excel through pivot tables but I want to learn how to do it in R. I'm sure it's also even more trivial in R if you know what you're doing lol.

Please see the image here of the calculation I want to do:

example

You can see from the screenshot here the white columns on the left are from my original data set and on the right two columns highlighted in blue the calculation I want to do.

The way (I think) the code should work is : group by election year, district #, what is the Dem % of vote, what is the Rep % of vote and then mutate a new column with the result of the "Dem margin" for each district.

I can then pivot this data into a new table which will show the Dem margin for each district, and from there be able to calculate average for the entire country and further calculations.

I think maybe I need to do this with a Ifelse statements nested inside of a mutate, but as a newb I'm having trouble how to format the code there.

Hopefully that makes some sense. Any ideas or someone willing to set me down on the right track?