Donald Trump's anti-trade stance has been blasted by a pillar of the corporate establishment in an attack reflecting growing alarm in business over the property mogul's dominance in the Republican presidential race.
"The Trump discussion on trade . . . is a very dangerous discussion," he told a Washington conference, making his first comments on the election since stepping down as Boeing's chairman at the end of last month.
"I'm not sure [about] putting up walls. The last time we did that was at the beginning of the Great Depression in the 1930s. So it should be done with a great deal of thought. I'm not sure that it's being that thoughtfully dealt with right now."
With a protectionist bent designed to woo angry blue-collar workers, Mr Trump has threatened to rip up trade deals, impose punitive tariffs on Chinese imports and force companies such as Apple to bring manufacturing back to the US.
Tom Donohue, head of the US Chamber of Commerce, was equally scathing about Mr Trump's proposals. "It's just pretty sort of stupid," he said, drawing laughs from a crowd of aviation executives as he spoke alongside Mr McNerney.
Their comments point to the depth of concern among big business, which can traditionally depend on the Republican presidential nominee to defend its interests, but is instead being assailed by this year's frontrunner as part of a failing US establishment.
While lobbyists in Washington are reluctant to express their worries publicly, Mr McNerney is newly liberated having left Boeing after 11 years at the top.
He was previously chief executive of the manufacturer 3M and a senior executive at GE and is still a director of Procter & Gamble and IBM. This month he joined Clayton, Dubilier & Rice, a buyout group, as a senior adviser.
"The precipitousness of the political debate is a little scary right now," Mr McNerney said.
Global trade, he said, was "a critical part of this country's prosperity and the growth and wellbeing of not only our citizens but citizens around the world".
But Mr McNerney said things on the campaign trail could get better. "I ultimately have faith in the political process in this country, believe it or not. If you ever weren't going to have faith, now's the time. But I have a feeling that emerging from this in some form or fashion will be a more centrist point of view."
His comments came after Doug Parker, chief executive of American Airlines, poked fun at Mr Trump on stage — something a corporate leader would not typically do in the middle of a presidential campaign.
Mr Parker said he was relieved that Mr Trump failed in a bid to buy American Airlines in 1989 and argued that one reason the businessman's Trump Airlines failed was that it foolishly offered to refund customers whose flights were delayed by more than 15 minutes.
When the Financial Times asked Mr Parker afterwards if he had concerns about Mr Trump's candidacy, he said: "I can't. I'm sorry. I'm not doing interviews."
With a variety of Republican efforts under way to stop the frontrunner from securing the nomination, the FT asked Mr Donohue of the Chamber of Commerce if he was involved.
"We don't do presidential politics. We do presidential policy," he replied. "We'll comment on some of the things he says, but we're not going to get involved in that other stuff."