WASHINGTON — Emerging signs of weakness in major economic sectors, including auto manufacturing, agriculture and home building, are prompting some forecasters to warn that one of the longest periods of economic growth in American history may be approaching the end of its run.

The economy has been a picture of health, expanding at a 3.5 percent annual pace during the third quarter and driving the unemployment rate to 3.7 percent, the lowest level in almost half a century. But General Motors’ plan to cut 14,000 jobs and shutter five factories reinforces other recent indications that the better part of the expansion is now in the rearview mirror.

“We’re in the 10th year of the expansion and there are some soft points,” said Ellen Hughes-Cromwick, a former chief economist at Ford Motor Co. and the Commerce Department who is now on the faculty at the University of Michigan. “The auto sales cycle has peaked and the housing cycle also has peaked.”

Ms. Hughes-Cromwick said higher interest rates, combined with rising inflation and faltering corporate confidence, could set the stage for a recession. In that scenario, she said, “I don’t really see how the economy can keep powering ahead.”