OPINION  — The topic never pops up in statistical analyses or pundit roundtables on cable TV, but one of the most underappreciated factors shaping politics is overconfidence.

Historically, second-term presidents have been particularly vulnerable to arrogant overreach. For eight decades, the prime example has been Franklin Roosevelt’s ill-fated plan following his 1936 landslide re-election to pack the Supreme Court with six new justices. (A personal plea: Please don’t mention this scheme to Donald Trump.)

Certainly, the 2016 Hillary Clinton campaign deserves its own Measuring-the-Drapes Wing in the Overconfidence Hall of Fame.

Perhaps a last-minute swing through Wisconsin and Michigan might not have made a difference since Clinton rallies were often flat and lifeless. But what remains baffling was the decision by the imperturbable Clinton high command to curtail state polling during the closing weeks of the campaign.

All this brings us to the most laughable manifestation of overconfidence in the 2018 campaign. It comes courtesy of an internal Republican National Committee poll obtained by Joshua Green of Bloomberg Businessweek.

The survey, conducted in early September, found that 57 percent of devoted Trump supporters believe there is absolutely no chance the Democrats can take back the House. As the accompanying report, written by the polling firm Public Opinion Strategies, bluntly states, “We need to make real the threat that Democrats have a good shot of winning control of Congress.”

Think about this for a moment. Convinced that polls are rigged for the Democrats (remember 2016) and conditioned by a president who denounces all criticism as “Fake News,” strong Trump backers have convinced themselves that the Republican Congress is an impregnable fortress. Of course, conditioned by Fox News, the minds of many of these MAGA-hat Republicans are also an impregnable fortress.

Maybe this RNC poll just captures a blip — and by November, Trump supporters will come to grasp that GOP majorities in the House and maybe the Senate are imperiled. But, at the moment, the survey illustrates how Trump’s insatiable bragging and his unrelenting war on reality threatens GOP turnout.

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Dropping the ball

A strong case can be made that the Republican congressional leadership has also overplayed its hand ever since Trump took office. Mitch McConnell and Paul Ryan cannot be directly blamed, of course, for Trump’s well-documented ignorance, his obsession with Fox News, his vicious attacks on the FBI, his constant belittling of his own attorney general and his obtuse refusal to understand or even read the Bill of Rights.

But in calculating their own response to the tumult and tantrums of Trump World, McConnell and Ryan gambled that fealty to the Great Leader represented smart politics for the Republican Party. Now, less than seven weeks before the election, it is looking as if the GOP congressional leadership was wildly overconfident about the degree to which Trump is a political asset when he is not running against Hillary Clinton.

Remember that until maybe six months ago, GOP leaders were convinced that the massive tax cuts and the booming economy would limit Democratic gains this year. Instead, in a world of Trumped-up diversion, the economy has faded as an election issue. A Gallup national survey last month found that voters are more concerned about “poor leadership” in Washington and related issues than the direction of the economy.

Even though the economy receives high marks in a recent Quinnipiac University National Poll — 69 percent of independent voters described it as excellent or good — the verdict on Trump himself should panic GOP leaders. Only 30 percent of voters describe Trump as “level headed” and, more worrisome, just 48 percent of the electorate believes the president is “mentally stable.”

All of this was predictable from the moment Trump took office and, despite many Republican fantasies, failed to magically become presidential. Fearful of primaries and losing Trump’s base voters (some of them very base), the GOP congressional leadership forgot about balanced budgets, free trade and standing up for law enforcement agencies.

In effect, the Republicans became unmoored as the party of Trump tweets.

Ryan, of course, took one look at the future on Capitol Hill and headed for the Exit sign.

A supreme solution?

But McConnell had another overweening strategy to save his majority. By rushing Brett Kavanaugh through the Senate in September, McConnell calculated that he would force endangered Democratic senators like North Dakota’s Heidi Heitkamp to choose either party loyalty (and campaign cash) or her conservative electorate.

In late August — weeks before Christine Blasey Ford came forward — Amy Walter of The Cook Political Report offered one of the smartest and most succinct summaries of the 2018 campaign. As she put it in a tweet, “A good rule of thumb when evaluating how an issue/event will impact R’s in the midterms. Does it help GOP reduce yawning gap they have w/ white, suburban women and indies? If it doesn’t, then it’s bad.”

No matter how the Kavanaugh nomination plays out from here, it is impossible to concoct a scenario under which the fight helps the Republicans with female voters. By Election Day, we may no longer be talking about a “gender gap” but rather discussing a “gender chasm” so large that it could swallow continents.

In the end, it comes down to Republican arrogance in believing that Trump could ever be tamed. What in Trump’s long history convinced GOP insiders and strategists that our reality-show president cares about anything not visible in his bathroom mirror?

Republicans are quickly learning that in politics — as in lion taming — nothing is as dangerous as misplaced overconfidence.

Walter Shapiro, a Roll Call columnist since 2015, has covered the last 10 presidential campaigns. He is also a fellow at the Brennan Center for Justice at NYU and a lecturer in political science at Yale. Follow him on Twitter @MrWalterShapiro.