Americans have voted former President Donald J. Trump back into the nation’s highest office four years after he fomented a riot at the Capitol to try to block his removal from power.

His election is likely to again place the country’s democracy under enormous stress. For the last decade, he has demonstrated that he has little regard for the checks and balances that have defined American government since the dawn of the republic.

Now, after Mr. Trump’s defeat of Vice President Kamala Harris, the nation faces momentous changes certain to cut across political and cultural lines. Republicans demonstrated strength all over the map and up and down the ballot. They seized control of the Senate and could retain the House. Winning both would give Mr. Trump an important source of legislative power.

As it was during Mr. Trump’s first term, the nation is set to be governed at the whims of a president with little interest in the details of policy. But under the government he assembles, major issues like abortion rights, taxation, immigration and foreign policy will be pushed hard to the right, from not only legislation and executive orders but also the inevitable appointment of Trump-friendly judges and, potentially, more Supreme Court justices.

Here are 10 takeaways from an election that again upends American politics.

America’s democracy is likely to be put under tremendous strain.

Mr. Trump has already sought to undermine the independence of the justice system. During his first term, he demanded personal loyalty from officials across the executive branch and fired those who resisted his demands.