What factors usually re-elect or throw out incumbent presidents?

The economy counts most.

Recessions, or at least chronic economic pessimism, sink incumbents. Presidents Jimmy Carter and George H.W. Bush were tagged with sluggish growth, high unemployment and a sense of perceived stagnation — and were easily defeated.

The 2008 financial crisis likely ended any chance for John McCain to continue eight years of Republican rule. Barack Obama campaigned on the message that incumbent George W. Bush was to blame for the meltdown and that McCain, his potential Republican successor, would be even worse.

A once-unpopular incumbent Ronald Reagan fought recession for three years. Yet he soared to a landslide victory in 1984 only after the gross domestic product suddenly took off at an annualized clip of over 7 percent prior to the election.

President Donald Trump’s economy is still booming. But his opponents here and abroad are counting on a recession to derail him.

They hope that either the good times can’t last forever or that Trump’s trade war with China will scare investors and businesspeople into retrenchment. Or perhaps massive annual deficits and staggering debt will finally catch up to a financially reckless government.

China will do all it can to prompt a U.S. downturn before November 2020 in hopes that it can get a better deal from a new Democratic president.

Unpopular optional wars are just as lethal to incumbents. Vietnam ended any chance of Lyndon Johnson seeking re-election. Iraq sank the second term of George W. Bush and almost cost him his 2004 re-election bid. The Benghazi fiasco, the collapse of Iraq and the rise of ISIS during Obama’s first term all made 2012 a far closer race than expected.

So far, Trump has been careful to avoid optional wars, nation-building and even so-called “police actions.” North Korea and Iran both know that all too well. So, they are likely to push the envelope in the expectation that either Trump will have to backpedal in fear of defeat in 2020, or that his tough stance will disappear with the election of a more accommodating Democratic president.

Scandals also can ruin re-election bids and second presidential terms.

Richard Nixon’s second term was cut short by Watergate. An impeached Bill Clinton lucked out that the Monica Lewinsky episode occurred after his successful re-election. Had the Iran-Contra scandal come to light in 1984 instead of 1986, Reagan might not have been re-elected in a landslide.

The 22-month Mueller investigation of “collusion” and “obstruction” proved a big dud. So too were serial efforts by Democrats to cut short Trump’s first term.

Over the next 14 months, we may we see a quite different news cycle in which Trump’s chief accusers — John Brennan, James Clapper, James Comey and Andrew McCabe — are cited for improper or even illegal conduct in their efforts to undermine the Trump campaign, transition and presidency.

Elections are not popularity contests. If they were, Trump might well lose handily, given that his approval ratings are consistently below 50 percent. Instead, they are choices between good and better — or bad and worse — candidates.

So far, the Democratic debates have been a great gift to Trump. The front-runners appear almost unhinged in promoting issues that that are not supported by a majority of Americans in polls. Those who sound moderate and centrist are either fading or, in the case of Joe Biden, face issues of competency, consistency and age.

Then there remain the known unknowns. Anything can happen before November 2020, from a hurricane to a third-party candidate.

Trump, our first president without either prior military or political experience, will remain a volatile candidate. He seems intent on replying to attacks without restraint through take-no-prisoners Twitter retorts, some of which turn off swing and suburban voters.

Yet no pundit has figured out whether Trump’s Twitter storms are the key to revving up his base in swing states, and thus might earn him another Electoral College victory without winning the popular vote, or if they finally will become too much for fence-sitting voters.

Finally, candidates have to campaign. Some, like supposed 2016 shoo-in Hillary Clinton, do it more poorly than others.

Trump will have lots more money this time around. And he will now act like a veteran on the stump.

Trump will turn 74 in 2020. But his near-animal energy belies his age. Some of his potential opponents — Biden, Bernie Sanders and Elizabeth Warren — are in their 70s and seem to show their age more than Trump does.

Add up all these factors, and a currently unpopular Trump will still likely be harder to beat than his confident media detractors and enraged progressive critics can imagine.

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