WASHINGTON — When in his 1964 GOP acceptance speech Barry Goldwater declared that “extremism in the defense of liberty is no vice,” a reporter sitting near journalist/historian Theodore White famously exclaimed: “My God, he’s going to run as Barry Goldwater!”

Six weeks into Donald Trump’s general election campaign, Republicans are discovering that he indeed intends to run as Donald Trump. He has boasted that he could turn “presidential” — respectful, respectable, reticent, reserved bordering on boring — at will. Apparently, he can’t.

GOP leaders who fell in line behind Trump after he clinched the nomination expected, or at least hoped, that he would prove malleable, willing to adjust his more extreme positions and tactics to suit a broader electorate.

Two problems. First, impulse control: Trump says what he actually feels, whatever comes into his head at any moment. Second, a certain logic: Trump won the primaries Sinatra-style, his way — against the odds, the experts and the conventional rules. So why change now? “You win the pennant,” Trump explained, “and now you’re in the World Series — you gonna change?”

Hence his response to the Orlando terror attack. Events like these generally benefit the challenger politically because any misfortune that befalls the nation gets attributed, fairly or not, directly or indirectly, to the incumbent party (e.g., the 2008 financial collapse). And Hillary Clinton is running as the quasi-incumbent.

The textbook response for the challenger, therefore, is to offer sympathy, give a general statement or two about the failure of the incumbent’s national security policy, then step back to let the resulting national fear and loathing, amplified by the media, take effect.

Instead, Trump made himself the (political) story. First, he offered himself unseemly congratulations for his prescience about terrorism. (He’d predicted more would be coming. What a visionary.) Then he went beyond blaming the president for lack of will or wisdom in fighting terrorism, and darkly implied presidential sympathy for the enemy. “There’s something going on,” he charged. He then reiterated his ban on Muslim immigration.

Why? Because that’s what Trump does. And because it worked before. It was after last December’s San Bernardino massacre that Trump first called for a Muslim ban. It earned him lots of opprobrium from GOP leaders and lots of support from GOP voters. He shot up in the polls, never to descend until he clinched. So why not do it again?

Because the general election is a different game. Trump assumes that the Republican electorate is representative of the national electorate. It’s not. Take the Muslim ban. Sixty-eight percent of GOP voters support it. Only 38 percent of Democrats do. And there are approximately 7 million more Democrats in the country. (Independents are split 51-40 in favor.)

The other major example of doing what’s always worked is the ad hominem attack on big-dog opponents. It worked in the primaries. Trump went after one leading challenger after another, knocking them out sequentially.

Hillary Clinton is a lousy campaigner but her machine is infinitely larger and more skilled than any of Trump’s 16 GOP competitors. More riskily, Trump is now going toe-to-toe with a sitting president.

Barack Obama is no Jeb Bush. He’s not low energy. He’s a skilled campaigner who clearly despises Trump and relishes the fight. And he carries the inestimable advantage of the gravitas automatically conferred by seven and a half years of incumbency. Moreover, he now enjoys an unusually high approval rating of around 53 percent. Trump’s latest favorability is 29 percent (Washington Post-ABC News).

It’s no accident that Trump’s poll numbers are sliding. A month ago, when crowned as presumptive nominee, he jumped into a virtual tie with Clinton. The polls now have him losing by an average of six points, with some showing a nine- and 12-point deficit (Reuters/Ipsos and Bloomberg).

This may turn out to be temporary, but it is a clear reflection of Trump’s disastrous general election kickoff. His two-week expedition into racism in attacking the Indiana-born “Mexican” judge. His dabbling in conspiracy, from Ted Cruz’s father’s supposed involvement in the Kennedy assassination to Vince Foster’s (“very fishy”) suicide. All of which suggests, and cements, the image of a man who shoots from the hip and is prone to both wild theories and extreme policies.

Reagan biographer Lou Cannon thinks that the Goldwater anecdote is apocryphal. How could anyone (even a journalist) have thought that Goldwater, who later admitted he always knew he would lose, was going to run as anything but his vintage, hard-core self?

Same for Trump. Give him points for authenticity. Take away for electability.

Charles Krauthammer’s email address is letters@charleskrauthammer.com.

Copyright 2016, The Washington Post Writers Group