"The probability that such a shift may occur during this particular Congress may well be even higher than that. At the moment, no fewer than six Democratic senators over the age of 70 represent states where a Republican governor would be free to replace them with a Republican, should a vacancy occur. Five other Democratic senators represent states for which a vacancy would go unfilled for months, until a special election to fill the seat was held — which would hand the G.O.P. control of the Senate at least until that election and likely for the rest of the current Congress if a Republican wins that contest. (In the case of Wisconsin, such a vacancy might not be filled until 2023.) All things considered, the odds that Democrats will lose control of the Senate in the next 22 months are probably close to a coin flip."
From "Justice Breyer Should Retire Right Now/If he doesn’t, Democrats run the very real risk that they would be unable to replace him" by lawprof Paul Campos (NYT).
