The data's level of measurement is nominal. An average cannot be taken of party affiliations, even if they are converted to numbers, because they have no numerical value and an average cannot be interpreted to mean anything. An alternative method in which an average can be taken is measuring political ideology on a numerical scale from most liberal to most conservative.
Work Step by Step
The level of measurement is nominal because party affiliation is categorical and each category has no numerical relationship with each other and cannot be ordered.