Currently the Republicans are at a historic high point and the Democrats are at a low point. The Republican dominance isn't guaranteed. Sixteen years ago Republicans controlled the White House and both houses of Congress. They managed to lose this in wave elections in 2006 and 2008, ending talk of a permanent Republican majority. With Barack Obama's election, the Democrats controlled government and the began to dream of a permanent Democratic majority. There was talk that demographics and a national swing to the left doomed the Republicans to being nothing but a regional rump party. 2010 and 2014 turned into Republican wave elections. The predicted 2016 Democratic wave never materialized. There are different ways of looking at this.
One is that American politics regularly swing back and forth. The White House certainly does. Since FDR, the White house has changed parties every eight years. The only exception to that was Ronald Reagan who defeated a sitting president and whose vice-president succeeded him with a convincing majority.
The party that controls the White House often looses support in Congress during the mid-terms so the Democrats may be one election away from starting to recover. If we assume that the national spirit is sort of a pendulum then it is likely that the Democrats will retake Congress during the Trump administration and will take the White House in 2024.
There is another possibility. Democratic control during the Obama administration may be an aberration caused by anger at the Iraq war in 2006 and the financial melt-down in 2008. In fact, a strong case can be made for this.
In theory the House of Republicans is the most sensitive to public mood. Every member stands for election every two years. In practice, change is slow. The Democrats controlled the House for decades from 195l to 1994. The Republicans began making gains under Reagan and finally took Congress in a wave election in the wave election during Bill Clinton's first mid-term. They held both houses of congress until George W. Bush's second mid-term in 2006. They started making gains again, taking the House in 2010 and retook the Senate in 2016. Looked at this way, the long-term swing has been Republican.
How does this work?
The country has become increasingly polarized. The coasts and cities are strongly liberal and the rest of the country is conservative. America is developing two cultures with little in common with each other. Regardless of demographics that are supposed to favor Democrats, it hurts them to be so tightly clustered. For all the talk about gerrymandering, it's very difficult to draw competitive districts when part of the population is widely spread out and part is tightly clustered.
It's been a truism among the Democrats for more than a decade that the Republicans have moved so far to the right that President Reagan would no longer be welcome. The truth is that both parties have changed over the years but it's instructive how they changed.
Reagan set the tone for the modern Republican party - a mixture of social conservatism and libertarianism with strong national defense. It's widely believed that President George H W. Bush's tax increase cost him reelection so Republicans have been anti-tax ever since. George W. Bush introduced "compassionate conservatism" meaning support for a strong safety net. He was influenced by no-conservatives who were socially moderate and strong on defense. Under Bush, the Republicans abandoned fiscal prudence and spent heavily. The TARP and President Obama's spending shocked the Republicans out of that, giving rise to the Tea Party movement. This was largely a return to the principals that Reagan ran on in 1980 with a heavy dose of social conservatism thrown in. Yes, the Tea Party was unforgiving of the sorts of compromises that President Reagan made but they would have welcomed candidate Reagan. The Tea Party eventually merged with the party in general only to run into Domald Trump's populism. What the results of this will be on the party have yet to be seen.
In contrast to the Republicans zig-zags, the Democrats have had a pretty straight trajectory. Bill Clinton was part of a moderate movement but since his election the Democratic party has moved further to the left than at any point in its history since FDR traded tips with Mussolini. They no longer even call themselves liberals. They are now progressives. This is an important distinction.
During the early 2000s, the Democrats made an effort to recruit moderates. This paid off in the wave election in 2006. But after that they lost the moderates. Some were purged and the rest lost their seats to Republicans. By the 2016 election, the party had moved to far to the left that a socialist was seen as a viable candidate. Hillary Clinton was actually running against Bill Clinton's biggest achievements. The party has moved so suddenly and so sharply to the left that candidate Barack Obama would be unacceptable. In 2008, Obama claimed to be against gay marriage and neutral on guns. Those positions have no place in today's Democratic party. He even made fun of Hillary Clinton for suggesting a manditor health care system comparable to Obamacare.
Today's Democratic Party places environmentalism and identity politics ahead of anything else. It has embraced the Black Lives Matter movement which regularly calls for killing cops. It is willing to put thousands out of work in the name of global warming, even if it's only a symbolic gesture. The hip city-dwellers are contempuous of blue collar workers and their values. They seem to attack every part of American life. Even the concept of gender is being eliminated with 32 or more genders recognized.
The Democrats have embraced identity politics. Individuals no longer matter, all that matters is the group they are part of. The assumption is that they can cobble together a winning coalition by appealing to various ethnic groups, particularly women, blacks and Hispanics. Even after their defeat in 2016, they believe that demographics are the key to their future political domination. This is quite a gamble and may not work. The groups they are counting on came together for Obama but not Clinton. It may well be that the first black president whose father was a foreign national was a once-in-a-lifetime uniter and that his coalition won't be duplicated.
The Democrats have one final problem. They have been too successful. They have a national health care. Obamacare may be replaced but it will not be removed. They also have gay marriage and a number of other achievements. Naturally, they have new goals but those have little support. Free college and an end to gender will not take back Congress. Even gun control has lost its edge since some states like California have enacted strict new gun laws that will blunt the desire for national action.
It is a given that the Democrats will oppose everything that Trump does but it's questionable if the country will follow them or if they will be willing to moderate some of their more extreme positions. Considering that Ralph Ellison, one of the furthest left members of Congress, is the next likely leader of the party, they seem set to follow the English Labor Party's march to the left and irrelevance.