Lee Drutman wrote a Vox article about how partisanship is currently threatening our separation of powers system.
Because the president is the only actor in the system who runs for office nationally, he has historically defined the party brand. And because the electoral fate of congressional partisans is linked to the brand of the party, they have a strong interest in going easy on fellow partisan presidents, while being tough on opposing partisan presidents. As a result, separation of powers has long been a dead letter without divided government.
Think of all the important moments when Congress has meaningfully checked abuses by the executive branch: Watergate, the 1975 Church Committee on wide-ranging domestic spying abuses by the CIA and the FBI, the Iran-Contra hearings. These were all moments of divided government, with Democrats in Congress and Republicans in the White House. Also note: The only two impeachment votes taken in Congress (Andrew Johnson in 1868, Bill Clinton in 1998-’99) came when Republicans controlled Congress and Democrats controlled the White House. None of these notable separation of powers moments would have taken place under unified government.
One might partially object to the above statement, and note that the courts, the third branch, have checked some of the Trump administration’s plans, notably the travel ban. But primarily, it has been liberal justices challenging the Trump administration. The conservative Supreme Court gave the administration the thumbs-up on the ban. And if the Trump administration succeeds in its efforts to remake the courts by appointing conservative justices, does anybody expect them to challenge the administration? The courts are more polarized than ever too.
Of course, the framers never anticipated this problem. Again, they thought American government would work without parties. And yet: It took a single Congress and Madison’s Republican Party was doing battle with Hamilton’s Federalist Party, voting in predictable patterns, calling each other nasty names (like “monocrat” and “Jacobin”), and fighting over the provision of presidential furniture. Whatever barriers Madison the framer enacted to make it hard for parties to form, Madison the partisan Congress member quickly found a way around them.
Perhaps the framers should have anticipated this. But they were humans, like everyone else. They couldn’t anticipate everything. And they were optimistic that they had designed a system of government that would frustrate partisan majorities from forming, solving the complex problem of divisive factions in a new and original way.