Messy new Congress and coming gridlock
With Democrats holding the narrowest possible majority in the Senate (one seat) and Republicans holding a slim majority in the House of Representatives (nine seats out of 435), you would expect to see the two parties working together. More likely, we will see even greater polarization in Congress, with conservatives wielding greater influence among Republicans and a more assertive progressive left in the Democratic Party.
That's because more and more states and districts are dominated by one party, owing to geographic polarization of the voters and, in the House of Representatives, redistricting to protect incumbents. The number of "battleground" states and districts has declined. In districts and states dominated by one party, the only real competition is in primaries. For incumbents, the threat of defeat usually comes from more extreme candidates - right-wing Republicans and left-wing Democrats. Few members of Congress are willing to take the risk of collaboration with the other party ("the enemy" to hard-core partisans).
Moreover, the U.S constitutional system makes it difficult to get anything passed. It's easy to block legislation in a complex and ungainly system, with two houses of Congress, three branches of government and competing centers of power in the federal government and the states. (The filibuster, a Senate rule that is not mentioned in the Constitution, is also a point of blockage.)
The U.S. system was designed to make it difficult to govern. The framers of the Constitution had just waged a revolution against a king. To them, strong government meant despotism. The Constitution replaced an earlier document, the Articles of Confederation, which had created a government that was so weak it was unworkable.
The idea was to limit power. The result is a constitutional system that works exactly as intended. Which is to say, it doesn't usually work very well at all. It's a plan for weak government and limited power, which is what the Founders wanted (and most Americans still do).
As president after president has discovered, there are innumerable ways opponents can stop things from passing, even if the president's party holds a majority in Congress. Look at what happened to President Clinton's health care plan in 1994, when the president's party controlled Congress. And how difficult it was for another Democratic Congress to pass Obamacare in 2010.
Deep polarization between Republicans and Democrats usually results in gridlock. In a parliamentary system like that of Britain, gridlock is unconstitutional. A core principle of the British constitution is, "His majesty's government must be carried on." If the government is gridlocked and cannot act, it falls - and new elections are held until the people elect a government that can act decisively. The United States has no king. There is no constitutional necessity for the government to rule decisively. And it often can't - to the consternation of many voters.
The miracle is that things do get done, often with impressive speed and efficiency. What's needed is a crisis. A crisis provides a powerful force of public urgency that lubricates the system and overwhelms blockages.
The Constitution was aimed at insulating government from public opinion, but public opinion - when it is rallied and mobilized - is the force that makes government work. Public opinion can overcome gridlock. It's the dirty little secret of American government: It works best when a crisis generates overwhelming public urgency.
The Great Depression was just such a crisis. It produced the outpouring of legislation that became the New Deal. The Cold War crisis produced the federal interstate highway system and, with the passage of the National Defense Education Act in response to the Soviet launching of the first space satellite, unprecedented federal involvement in education.