Here is Sean Trende's typically in-depth, by-the-numbers analysis of what it means for an election to be a referendum on the incumbent, and whether this presidential election will be such an election.
His conclusion is in his title.
Perhaps this is a good time to remind you of why, though I test solidly Libertarian in every quiz on political identification I have ever taken, I am so consistently Republican in my political choices and actions.
As you know, my focus is on regaining our lost economic liberty, including the right to make a living in whatever fashion and at whatever pay we can negotiate with those we serve, as well as the capitalist right to pursue happiness and keep what we catch.
The Democrats now openly call themselves Progressives, because they seek to make "progress" toward an economy with central control radiating outward from all levels of government; that is, they seek a return to the aristocratic economies of 18th century Europe from we fought our way clear. The same European notion of central planning they advocate progressed through the 19th century industrialization, culminating in the 20th century Fascist, National Socialist, and Communist tyrannies. If you want to understand the trajectory they wish us to follow, read Friedrich Hayek's Road to Serfdom.
While the Republican party is not unerring in favoring individual economic liberty, it is by far the better choice for those who seek more economic freedom. That is to say, there is simply no intellectual justification for any thinking advocate of economic liberty to support a Democrat candidate against a Republican one.