Sort of begs the question doesn't it? What is a republican?
A member of the Republican Party is generally considered a Republican.
Is it distinct from a democrat, if so how distinct?
Generally, Republicans are people who believe in some or all of the following - fiscal accountability, free-market economics, social responsibility, respect for life, low taxes, smaller government, gun rights, and integrity in government.
Arnold apparently buys into the economic ideas of the agenda, though not the social aspects.
McClintock buys into the whole agenda, but only gathers 18% support, which is the functional equivalent of 0% support. It would have been wise for him to work with the moderate wing of his party, so he could turn to them for support in the future. Arnold as governor and McClintock as Senator would have been a favorable outcome for all Republicans. But McClintock chose to go down with his governor-ship instead.
That was a very poor decision that did much harm to the conservatives in California. They are now far more marginalized than ever.