Mark Weinberg is a communications consultant, speechwriter and the author of "Movie Nights with the Reagans" (Simon & Schuster). He served as special assistant to the president and assistant press secretary in Ronald Reagan's White House, and as director of public affairs in President Reagan's office. The opinions expressed in this commentary are his own. View more opinion on CNN.

(CNN) On Monday, General Motors (GM) announced that it will close five plants in North America and cut thousands of jobs, marking one of the most serious economic challenges to the Trump presidency yet.

Until now, it appeared President Donald Trump had done a good job of energizing the American economy, specifically when it came to jobs. Legitimately or not, he has been praised for record low unemployment -- now at 3.7% , a 49-year low. While liberal critics argue that he is merely the beneficiary of the Obama administration's smart economic policies, it is hard to definitively say what the driver is.

Mark Weinberg

What we do know is this: Had unemployment increased while Trump was in the White House, he would have been blamed. So, it seems only fair to give him some credit for low unemployment numbers now.

But that credit may not sustain him for long. While GM has said that an evolving marketplace and changing consumer demands are the chief reasons for the actions it took, the Trump administration's onerous steel tariffs are also an important contributing factor. It's more expensive to buy steel parts for cars than it was before he took office.

That Trump's economic policies are in any way causing manufacturing plants to close and jobs to be eliminated could be a stake in the heart of the Trump presidential experiment. In response, on Tuesday, the President, clearly concerned by GM's announcement, threatened to cut the car company's subsidies.