under the plan that would have been carried by each political party’s candidates for the office of President and the office of Senator in the 2 most recent general elections for the office of President and the 2 most recent general elections for the office of Senator (other than special general elections) immediately preceding the enactment of the plan, except that if a State conducts a primary election for the office of Senator which is open to candidates of all political parties, the primary election shall be used instead of the general election and the number of districts carried by a party’s candidates for the office of Senator shall be determined on the basis of the combined vote share of all candidates in the election who are affiliated with such party. (ii) Determine, for each of the 4 elections assessed under clause (i), whether the number of districts that would have been carried by any party’s candidate as determined under clause (i) results in partisan advantage or disadvantage in excess of the applicable threshold described in subparagraph (C). The degree of partisan advantage or disadvantage shall be determined by one or more standard quantitative measures of partisan fairness that— (I) use a party's share of the statewide vote to calculate a corresponding benchmark share of seats