For almost three months, Tesla reigned as the most valuable automaker in the nation, ahead of both General Motors and Ford Motor, thanks to a remarkable run-up in its stock this year. It seemed its chief executive, Elon Musk, could do no wrong.

But a spate of negative news this week has brought the electric-car maker’s many challenges into focus, especially its ambitious plans to ramp up production of its first mass-market offering, the Model 3, which begins rolling off the assembly line on Friday.

With investors shying away, Tesla has seen its shares fall by almost 17 percent since Monday’s intraday high, to $308.89, including a 5.6 percent drop on Thursday. That reduced the company’s market capitalization to $50.7 billion, according to Bloomberg, and put G.M. ahead once again, at $52.6 billion.

Most critical to Tesla’s troubles was a new, slower timetable for the Model 3 introduction, and a separate acknowledgment of problems manufacturing battery packs, a critical component of its cars, in the second quarter, said David Whiston, an analyst at Morningstar.