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24 

RUCHO v. COMMON CAUSE 

Opinion of the Court 

party’s advantage may be shown through sensitivity test-
ing:  probing  how  a  plan  would  perform  under  other  plau-
sible  electoral  conditions.”  Brief  for  Appellees  League  of 
Women  Voters  of  North  Carolina  et al.  in  No.  18–422, 
p. 55.  See also 318 F. Supp. 3d, at 885.  Experience proves
that  accurately  predicting  electoral  outcomes  is  not  so 
simple,  either  because  the  plans  are  based  on  flawed
assumptions  about  voter  preferences  and  behavior  or 
because demographics and priorities change over time.  In 
our  two  leading  partisan  gerrymandering  cases  them-
selves, the predictions of durability proved to be dramati-
cally wrong.  In 1981, Republicans controlled both houses
of  the  Indiana  Legislature  as  well  as  the  governorship.
Democrats challenged the state legislature districting map
enacted  by  the  Republicans.    This  Court  in  Bandemer 
rejected  that  challenge,  and  just  months  later  the  Demo-
crats  increased  their  share  of  House  seats  in  the  1986 
elections.  Two  years  later  the  House  was  split  50–50 
between  Democrats  and  Republicans,  and  the  Democrats
took  control  of  the  chamber  in  1990.  Democrats  also 
challenged  the  Pennsylvania  congressional  districting 
plan  at  issue  in  Vieth.  Two  years  after  that  challenge 
failed, they gained four seats in the delegation, going from 
a 12–7 minority to an 11–8 majority.  At the next election, 
they flipped another Republican seat.

Even  the  most  sophisticated  districting  maps  cannot 
reliably account for some of the reasons voters prefer one 
candidate  over  another,  or  why  their  preferences  may 
change.  Voters  elect  individual  candidates  in  individual 
districts,  and  their  selections  depend  on  the  issues  that 
matter  to  them,  the  quality  of  the  candidates,  the  tone of 
the  candidates’  campaigns,  the  performance  of  an  incum-
bent, national events or local issues that drive voter turn-
out,  and  other  considerations.  Many  voters  split  their
tickets.  Others  never  register  with  a  political  party,  and 
vote  for  candidates  from  both  major  parties  at  different