Matthew Yglesias, in a nice post exploring the nature of presidential political capital, writes:
The American presidency is a weird institution. If Barack Obama wants to start a war with North Korea and jeopardize the lives of hundreds of thousands of people, it’s not clear that anyone could stop him. If he wants to let cold-blooded murderers out of prison, it’s completely clear that nobody can stop him. But if he wants to implement the agenda he was elected on just a few months ago, he needs to obtain a supermajority in the United States Senate.
I don't really see how this makes the presidency a weird institution--what it means is that presidential campaigns are very strange creatures. The reality is that we have a system of government in which domestic policy is by and large set by Congress. You might think this is a good thing or a bad thing--I tend to think it's a good thing--but it certainly isn't a new thing; it's the way the system has always worked. In a more rational world, presidential campaigns would focus exclusively on questions of foreign affairs, judicial appointments, how to run the administrative state, and so forth. Voters would laugh off the stage any presidential candidate pledging to reform entitlement programs or labeling herself the "commander in chief of the economy," and no campaign would bother putting out, say, detailed proposals for health care reform. It would be almost as ridiculous as a candidate running for the House of Representatives on a platform of overturning Roe v. Wade (though, come to think of it, I guess that happens a fair amount too).
I don't have any terrific explanation for why campaigns focus so intently on issues the victor will have minimal say over, except to observe that civic education in America isn't too great. Maybe policy proposals just end up being proxies for deeper political values--that's the role health policy white papers seemed to play in last year's Democratic primaries. Maybe the concept of separation of powers doesn't come too naturally to the human psyche; we simply assume anyone we elect is supposed to have some influence over whatever problems happen to be salient at the moment. But that doesn't strike me as a very strong normative argument against the separation of powers in the first place. If anything, it's the opposite--if your electorate can't even tell the difference between legislative and executive power, it's probably a good idea not to vest too much power in the victor of a single election for a single office.