In the traditional view, the unique American system of diffusing political power across the House, Senate, and Presidency creates an exceptional structure of checks and balances.  In Madison's vision, each of these institutions would have “a will of its own” so that “ambition [would] counteract ambition” to produce better outcomes.  But whether the system ever worked this way, the nature of modern political party competition has completely overwhelmed that original vision.  If you have any doubt about that, see Ezra Klein's piece in yesterday's Washington Post (here).

Presidents are the most significant representatives and leaders of one political party; when they make a policy issue theirs, it becomes a partisan political issue; through the linkage of partisan affiliation, the electoral fates of members of Congress are powerfully tied to the perceived success or failure of the President; thus, members of Congress are driven by partisan incentives and “wills,” not by any “institutional will” of the Senate or House.  This is dramatically accentuated in our era by the hyperpolarized nature of the political parties:  The President creates the cues and the political parties function, for the most part, as blocs.

Daryl Levinson and I explored in detail how the rise and polarization of the political parties has completely transformed the separation of powers system, in a Harvard Law Review article titled Separation of Parties, Not Powers.  For the details, see here.

Here's the key passages from Klein's piece:

This is perhaps the most important and least understood fact in today’s Washington: Presidents polarize. As the effective leader of one of two political parties, the president is inevitably a polarizing figure. . . That effect is not, from the president’s perspective, all bad. It makes it easier for him to corral members of his own party, as Obama discovered from 2009 to 2010, when Democrats controlled the House and Senate and passed the stimulus, health-care reform, Dodd-Frank financial regulations and much more. . . . But today’s intense polarization means that most any bill associated with Obama is automatically targeted for defeat by Republicans. Policy compromise, as the White House has found out again and again, isn’t enough to overcome the zero-sum world of modern politics.