President Trump at the final presidential debate on the campus of Belmont University on Thursday. (Jabin Botsford/The Washington Post)

MANY PEOPLE may find it hard to understand, but just over a week before the election, some voters remain undecided. To them we would say: A vote for a second Trump term is a vote for an America in decline and an American democracy in danger.

At best, the demise would be gradual — a descent into diminished prosperity, constricted opportunity for your children and grandchildren, waning influence overseas and continued erosion of democratic norms at home.

This is not a matter of conjecture; it is a judgment based on President Trump’s record and promises.

What are the sources of U.S. prosperity — of our ability to generate and enjoy more than 15 percent of the global economy with just over 4 percent of the world’s population? They include a predictable rule of law; a professional civil service; a position as global leader that lets us help set the rules and have the U.S. dollar accepted as the only true international currency; and high, if not world-leading, standards of health care and education.

Also key has been a broadly shared commitment to fairness and equal opportunity, even if we argue ferociously about how to translate that commitment into policy. We have prospered, while other developed nations have begun to stagnate, by attracting talented, entrepreneurial and ambitious immigrants from all over the world. Our commitment to freedom has allowed immigrants and native-born alike to contribute to the fullest extent of their abilities.

Mr. Trump would undermine all of those strengths. He replaces rule of law with presidential whim, picking and choosing corporate favorites and twisting the criminal system to favor his friends. At an accelerating pace, he is politicizing, corrupting and sapping the morale of our government — our foreign service, our health and scientific agencies, our keepers of statistics. Many will hesitate to invest — to build new factories or create new jobs — if law and governmental power become unpredictable, wielded to reward cronies and punish the disfavored.

He craves the approval of autocrats who wish our country ill while abandoning and insulting allies; the latter will not stand by and take his abuse for four more years, while the former will exploit his credulity. Already the United States finds itself humiliatingly isolated on key issues, like relations with Iran. As Mr. Trump fulfills long-held ambitions to undermine alliances with Europe, Japan and South Korea, the United States will be further enfeebled; China, increasingly dominant; the world, ever less stable.

He is in court seeking to overturn the Affordable Care Act, so that no one with a preexisting health condition could be sure of obtaining insurance; tens of millions of Americans could lose access to health care.He pretends to object only to undocumented immigration, but he has cut legal immigration in half. The most talented scientists and computer engineers of the next generation are choosing Canada, Australia, China — anywhere but Donald Trump’s America.

That America, in Mr. Trump’s vision, is one in which groups are pitted against each other, not encouraged to cooperate. States and cities with Democratic-leaning populations are enemy territory. He is contemptuous of any movement for equal justice and friendly to white supremacists. He has named 56 men and women to the nation’s highest courts—the Supreme Court and federal appeals courts. Not a single one is Black.

In Mr. Trump’s America, science and truth are treated with contempt. With his mangled response, the novel coronavirus has claimed more lives here than in any other country, and the pandemic and its accompanying recession could drag on long into a second Trump term. The contempt for science likewise shapes Mr. Trump’s utter failure to respond to climate change. The Earth is ailing; the damage from four more years of regression could be irreparable.

In Mr. Trump’s America, political rivals are traitors who must be prosecuted and jailed. Congressional oversight is an inconvenience that can be ignored and, eventually, suppressed. Journalists seeking to report on his administration are enemies of the people. He welcomes foreign interference to help his campaign, undermines confidence in the election and threatens not to accept its results. If he remains in power, fairly or fraudulently, there is no reason to believe that in a second term Mr. Trump will not act on his authoritarian impulses. His incompetence in government, though real, will be no protection; he has shown himself, in the past year, increasingly adept at evading the checks and balances we thought the Constitution guaranteed.

Finally: Mr. Trump has proven himself, in the covid-19 catastrophe, incapable of leading in crisis. What if the next virus is far more deadly — which health experts say is entirely possible? What if the next emergency involves a risk of nuclear war, given Mr. Trump’s abject failure to rein in the nuclear programs of Iran or North Korea? Can anyone trust him to manage such a challenge, atop an administration from which he has hounded almost all knowledgeable and experienced officials?

As we’ve written before, we believe former vice president Joe Biden well-suited to be president. You, undecided voter, may be less sure; maybe you disagree with some of the policies he espouses — that’s fine. We would simply ask you to weigh your concerns about the unknowns of a Biden presidency against the certain dangers of a second Trump term. On the one hand, a tax, a minimum wage, an energy policy you might not like; on the other, the demise of U.S. democracy, prosperity and global leadership. It shouldn’t be a hard call.