Among the unwritten rules that have sustained American democracy are partisan self-restraint and fair play. For much of our history, leaders of both parties resisted the temptation to use their temporary control of institutions to maximum partisan advantage, effectively underutilizing the power conferred by those institutions. There existed a shared understanding, for example, that anti-majoritarian practices like the Senate filibuster would be used sparingly, that the Senate would defer (within reason) to the president in nominating Supreme Court justices, and that votes of extraordinary importance — like impeachment — required a bipartisan consensus. Such practices helped to avoid a descent into the kind of partisan fight to the death that destroyed many European democracies in the 1930s.

Yet norms of partisan restraint have eroded in recent decades. House Republicans’ impeachment of Bill Clinton in 1998 abandoned the idea of bipartisan consensus on impeachment. The filibuster, once a rarity, has become a routine tool of legislative obstruction. As the political scientists Thomas Mann and Norman Ornstein have shown, the decline of partisan restraint has rendered our democratic institutions increasingly dysfunctional. Republicans’ 2011 refusal to raise the debt ceiling, which put America’s credit rating at risk for partisan gain, and the Senate’s refusal this year to consider President Obama’s Supreme Court nominee — in essence, allowing the Republicans to steal a Supreme Court seat — offer an alarming glimpse at political life in the absence of partisan restraint.

Norms of presidential restraint are also at risk. The Constitution’s ambiguity regarding the limits of executive authority can tempt presidents to try and push those limits. Although executive power has expanded in recent decades, it has ultimately been reined in by the prudence and self-restraint of our presidents.

Unlike his predecessors, Mr. Trump is a serial norm-breaker. There are signs that Mr. Trump seeks to diminish the news media’s traditional role by using Twitter, video messages and public rallies to circumvent the White House press corps and communicate directly with voters — taking a page out of the playbook of populist leaders like Silvio Berlusconi in Italy, Hugo Chávez in Venezuela and Recep Tayyip Erdogan in Turkey.

An even more basic norm under threat today is the idea of legitimate opposition. In a democracy, partisan rivals must fully accept one another’s right to exist, to compete and to govern. Democrats and Republicans may disagree intensely, but they must view one another as loyal Americans and accept that the other side will occasionally win elections and lead the country. Without such mutual acceptance, democracy is imperiled. Governments throughout history have used the claim that their opponents are disloyal or criminal or a threat to the nation’s way of life to justify acts of authoritarianism.

The idea of legitimate opposition has been entrenched in the United States since the early 19th century, disrupted only by the Civil War. That may now be changing, however, as right-wing extremists increasingly question the legitimacy of their liberal rivals. During the last decade, Ann Coulter wrote best-selling books describing liberals as traitors, and the “birther” movement questioned President Obama’s status as an American.