The Dow Industrials dropped more than 400 points, or about 1.5 percent, in morning trading. The blue-chip index followed the tech-heavy Nasdaq Composite, which was down 2.6 percent. The S&P 500 was down 1.6 percent and European indexes were in the red across the board.

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Markets had experienced declines before last week’s midterms, with some swings larger than that seen Monday.

The markets had one of their worst Octobers in years last month, with the Dow shedding 5 percent, the S&P dropping 7 percent and Nasdaq watching 9 percent vaporized at one point.

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The Dow plunged 1,300 points in one, two-day rout last month, including 800 points in one session, rattling investors.

Daniel P. Wiener, chief executive of Adviser Investments, a Newton, Mass.-based firm that manages more than $5 billion in assets, said Monday’s market volatility had nothing to do with politics.

“Donald Trump is getting anxious now,” Wiener said. “He is getting nervous. And so he will lash out and grab onto any disruption in the economy, in the market and in politics to try to deflect from the issues that are really critical to the country.”

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Fresh off last week’s midterm victories, Democrats have begun planning an array of investigations, vowing far more vigorous oversight of the Trump administration than during the past two years, when Republicans controlled the House.

Democrats, including Rep. Eric Swalwell, pushed back on Trump’s tweet Monday.

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“The days of Presidential Immunity are OVER. Gone,” Swalwell wrote on Twitter. “No more free passes to cash in on access to the Oval Office. No more colluding with Russia. And we’ll see if you’re a tax cheat. America is putting a balance of power on your abuses of power. Welcome to democracy.”

Trump’s tweet appeared to reference comments by Senate Majority Leader Mitch McConnell (R-Ky.). Last week, McConnell repeated a caution to Democrats that Republicans experienced a backlash for aggressive investigations during the tenure of President Bill Clinton.

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“The business of presidential harassment, which we were deeply engaged in the ’90s, improved the president’s approval ratings and tanked ours,” McConnell said during a news conference.

Some of the House Democratic plans were described during media interviews Sunday.

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Rep. Jerrold Nadler (D-N.Y.), who is poised to take control of the House Judiciary Committee, said he will call acting attorney general Matthew G. Whitaker as a first witness to testify about his “expressed hostility” to special counsel Robert S. Mueller III’s Russia investigation. Nadler said he is prepared to subpoena Whitaker if necessary.

Another incoming chairman, Rep. Adam B. Schiff (D-Calif.) of the House Intelligence Committee, raised the possibility of investigating whether Trump used “instruments of state power” to punish companies associated with news outlets that have reported critically on him, including CNN and The Washington Post.

And Democrats on the House Oversight and Government Reform Committee plan to expand their efforts to investigate Trump’s involvement in payments to women who alleged affairs with him before the 2016 election, a committee aide said Sunday night, potentially opening up the president’s finances to further scrutiny.