WASHINGTON — Weeks of turmoil in financial markets and an extraordinary outburst over trade from President Trump have opened a fresh lane of attack for Democrats hoping to unseat a president who has put the strength of the economy at the center of his re-election pitch.

On the campaign trail this past week, Democratic candidates blasted Mr. Trump for his economic stewardship, criticizing his gyrating trade policy and accusing him of hurting American workers and farmers and stoking a possible recession. In doing so, they signaled a readiness to seize an issue that has been a strength for Mr. Trump — the nation’s economy — and harness it as an advantage for themselves.

That strategy appeared to gain potency on Friday when the president attacked his own Federal Reserve chairman, escalated hostilities with China over tariffs and demanded that American companies stop doing business with the Chinese. It was a day of stunning economic tumult that sent stocks plunging, which the president then joked about in a tweet.

Democrats quickly cited the day’s developments to press the case that Mr. Trump was losing his grip on economic policy.