As far as party primaries are concerned, both Republican and Democratic Party primaries are dominated by the most zealous voters, whose views may not reflect the views of most members of their own respective parties, much less the views of those who are going to vote in the November general election.
In recent times, each election year has seen each party's nominee selected - or at least subject to veto - by its most extreme wing and then forced to try to move back to the center before the general election.
This can only undermine the public's confidence in the integrity of the candidates of both parties.

"Bipartisan Primary Blues", National Review (25 March 2008).