Mr. Manley said he did not expect a huge downturn like the one that hit the industry in 2008 and 2009. “But the reality is, the market is cyclical,” he said. “At some stage, the industry is going to drop off on its own.”

Most analysts see auto sales slipping only slightly in 2020, to a range of roughly 16.5 million to 16.9 million vehicles. “With the overall strength in the economy, with full employment, you’ve got consumer confidence at an all-time high,” said Jack Hollis, group vice president and general manager of Toyota in North America.

In addition, the three big American automakers bought four years of labor peace in the fall with new union contracts, though at the cost of a 40-day strike against General Motors.

Nevertheless, the industry still faces some risks. The Trump administration’s trade war with China remains unresolved. And the rising tensions between the United States and Iran are a reminder of the potential volatility of oil prices.

The new trade deal by the United States, Mexico and Canada is likely to increase domestic production and auto employment slightly and cause automakers to think hard before adding new plants in Mexico. “The question is whether it raises prices, and if that hits demand,” Mr. Wakefield said.

One worry is whether the steady rise in prices is sustainable, along with the increasing debt that owners have taken on to buy those more expensive cars.

More than one-third of Americans now have auto loans, up from 20 percent in 1999, according to the Federal Reserve Bank of New York. The share of consumer debt going to auto loans climbed to 9.4 percent in the third quarter of 2019, compared with 6 percent in the same period 10 years ago.