Employers created 37,000 new manufacturing jobs in July, the Labor Department said Friday, the strongest gain since December and the third straight month in which factory job growth accelerated.

The July numbers may have been nudged upward a bit by vagaries in the data due to annual auto plant shutdowns, but the trend has been firm for the past year and a half. The 327,000 new jobs over the last year represent the best 12-month stretch in 23 years.

Analysts point to several factors that have kept hiring on an upswing, even as they eye growing headwinds.

Related: U.S. gains 157,000 jobs in July as unemployment falls to 3.9%

“By and large, we’ve been in a period of the entire globe doing pretty well,” said Mark Muro, a senior fellow and policy director at the Brookings Institution whose work focuses on economic development, manufacturing, and innovations like digitization.

Manufacturing has also ridden a wave of recovery from the oil price shock that started in late 2014, and hearty demand for new motor vehicles.

In fact, the levels of the ISM manufacturing surveys over the past year or so would generally have pointed to even higher payroll numbers, said Jim O’Sullivan, chief U.S. economist for High Frequency Economics. O’Sullivan thinks weak productivity numbers have been offsetting the strong job gains and keeping output tepid.

But all good things must come to an end in the post-crisis economy, it seems. “In general, global growth has cooled a bit in the last six months,” O’Sullivan said.

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Motor vehicle sales have clearly flattened after years of blowout sales, he pointed out. And yet in the near term, O’Sullivan said, trade concerns are the biggest downside risk.

Muro put it this way: “The irony here is that just when things have gotten on a roll, a set of self-inflicted wounds—such as trade uncertainties and regulatory uncertainties on emissions—may introduce new headwinds.”

Those headwinds may be starting to show up, below the headlines but slowly and surely, in some economic data. The ISM manufacturing index for July stumbled, although it remained at a high level, with many survey respondents pointing to tariff uncertainties.

According to an analysis from Bespoke Investment Group, nearly two-thirds of companies classified as “industrial” mentioned the word “tariff” on earnings conference calls in the second quarter, while 57% of companies in the “materials’ sector, like PPG Industries PPG, +0.53% , mentioned the word.

There’s another big risk, not just for factory employment, but all types of jobs. “Over time as the Fed keeps tightening the goal is to slow employment growth down,” O’Sullivan said. “I think the Fed needs to see those numbers cut in half, but what exactly that means for manufacturing, there’s no formula.”

Brookings compiled a list of the top 10 states by contribution to 2016-2017 national manufacturing job growth. Only two states listed, California and Illinois, did not vote for Donald Trump.

State and rank Employment change November 2016-December 2017 Share of national growth 1. Texas 26,120 11.7% 2. California 16,226 7.3% 3. Indiana 15,138 6.8% 4. Michigan 14,905 6.7% 5. Georgia 11,896 5.3% 6. Illinois 10,808 4.9% 7. Florida 8,987 4.0% 8. Wisconsin 8,829 4.0% 9. Ohio 8,675 3.9% 10. Pennsylvania 8,379 3.8% (source: Brookings Institution of Labor Department’s QCEW data)

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