V = A + b1 Vt-1 + b2 Vt-2 + b3 DPRIM + b4 RPRIM
Vt-1:Incumbent party’s popular two-party vote in the last election
Vt-2:Incumbent party’s popular two-party vote in the second to last election
DPRIM:Incumbent-party (Democratic) candidate primary support in New Hampshire and South Carolina
RPRIM:Opposition-party (Republican) candidate primary support in New Hampshire and South Carolina
V:Incumbent share of the two-party vote
A:Constant
http://pollyvote.com/en/components/econometric-models/primary-model/