Many American families discourage talking politics at Thanksgiving dinner. In U.S. politics, meanwhile, there’s a powerful incentive not to talk about being thankful.

Competing politicians and political parties have no interest in cultivating a general sense of gratitude in the electorate. They tend to focus voters’ minds on what’s going wrong and to blame the other guy.

Up to a reasonable point, this is acceptable. The alternatives are complacency and the stagnation it breeds, or official suppression of discontent, the evil against which the founders of this country rose up.

They thought complaining was so essential to freedom, in fact, that they put the right to petition government for a redress of grievances in the First Amendment to the Constitution.

The American predicament this Thanksgiving can be summarized as follows: The normal and necessary ways of expressing and redressing grievance have gone badly awry. Society is consumed by negative partisanship. Restoring the right balance is the key to stabilizing the republic.

That will not be possible as long as the political culture is dominated by a president who indiscriminately hogs credit — a year ago he said he was thankful “for having made a tremendous difference in this country” — and casts blame, portraying himself as a victim of conspiracies and “rigged” processes while encouraging supporters to think and do the same.

How revealing that the injustice he perceived, and rushed to correct, in the pre-holiday week was the legal and administrative process against soldiers credibly accused by their own military services of wanton violence against defenseless people.

Four more years of Donald Trump’s corrosive presidency could do lasting harm to American institutions and to this country’s global standing, moral and strategic.

As history would have it, though, his first term took place during a time of significant and, seemingly, durable prosperity. There is simply no denying, or ignoring, this fact.

Unemployment is at a mere 3.6 percent, the lowest rate in 50 years. Real wages have been trending up for about half a decade. Stock indexes are at all-time highs. Short-term recession fears that rose earlier this year have dissipated.

Long-term structural threats remain — productivity growth, the government’s huge debt, inequality of wealth and income — and the trade war Trump started with China and other countries continues to chill investment.

For now, however, 56 percent of Americans said their personal finances were good or excellent and 69 percent expected them to improve in a year, according to a Gallup poll. Gallup’s “personal financial worry index” is at 46, the lowest level since just before the Great Recession.

And the rules of politics entitle Trump to reap public gratitude for the situation, even though it is hardly of his making, except insofar as the tax cut he and the Republicans enacted two years ago provided a short-lived stimulus.

Fifty-seven percent of the public approves of the president’s handling of the economy, while 42 percent disapprove, according to Gallup, a net rating of plus 15 points. He has not had a negative rating on the economy in two years.

In other words, the politics of thankfulness cut against the Democrats on one of the two issues that most concern voters (health care is the other), according to multiple surveys. This isn’t fair, necessarily, but it requires the president’s opponents to develop a message that reassures millions of people who like the way the economy is going.

This is what former president Barack Obama clearly had in mind when he told Democrats on Nov. 15 that “the average American doesn’t think we have to completely tear down the system and remake it” — contrary to the sweeping health-care, tax, housing and energy plans unveiled by Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), or to Democratic calls for executive action as aggressive as Trump’s own.

Trump’s appeal to the voters in 2020 will be simple: Keep me for four more years, and you will benefit materially; ignore all that fuss about truth, democratic values and the rule of law. It’s a Faustian offer that has worked for populist demagogues before.

A Democrat will have to convince the people that intangible values are essential to what we have to be thankful for as Americans.

In his Thanksgiving proclamation for 1934, President Franklin D. Roosevelt expressed gratitude for the nation’s turn “to things spiritual” in the previous year. “We can truly say, ‘What profiteth it a nation if it gain the whole world and lose its own soul?’ ” FDR wrote.

That was in the depths of the Great Depression, and Roosevelt was the incumbent. The task facing Roosevelt’s political heirs is to make the case that the same principle applies during times of plenty.

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