The president's incendiary assessment came in the middle of a speech designed to tout the benefits of the Republican tax-cut package, which Trump said set off "a tidal wave of good news that continues to grow each day."
As he spoke, Trump made no mention of the remarkably volatile day on Wall Street that ended with the Dow Jones industrial average plunging more than 1,150 points. Trump frequently cites the Dow's record rise over the last year as evidence that his tax cut and deregulation programs are working.
Though a White House spokesman told reporters beforehand it was not a political speech, the event in many respects resembled Trump's freewheeling campaign rallies at which he pingpongs from one subject to another.
Trump touted a Republican Senate candidate, Rep. James B. Renacci (Ohio), and reveled in the crowd's chant of "U-S-A."
Besides assailing the "dead silence" of Democrats at his State of the Union speech last week, Trump recounted his 2016 election victory in Ohio, talked up Republican prospects in the 2018 elections and referred to reports that Hillary Clinton's discarded smartphones were destroyed by hammers during her tenure as secretary of state.
Trump also praised the work of Rep. Devin Nunes (R-Calif.), whose controversial memo accusing the FBI of surveillance abuses was released last week with Trump's blessing.
"Oh but did we catch them in the act or what?" Trump said. "You know what I'm talking about. Oh, did we catch them in the act. They are very embarrassed. They never thought they were going to get caught. We caught them. We caught them. It's like so much fun, like the great sleuth."
In his State of the Union address, Trump touted rates of unemployment for blacks and Hispanics that he said were the "lowest in the history of the country."
Democrats have said Trump is taking undue credit for a trend that began under President Barack Obama. Since the recession's end, unemployment rates for all demographic segments have fallen, including blacks and Hispanics.
The black unemployment rate was at its lowest point in history in December after it was more than cut in half during the last three-quarters of Obama's presidency. The unemployment rate for black Americans actually jumped in January, according to Bureau of Labor Statistics data.
Trump said on Monday Democrats had decided in advance not to applaud.
"It was a game, you know they play games," Trump said. "They were told, don't even make a facial movement."
Trump said he noticed one lawmaker who was "putting his hands together."
"He was probably severely reprimanded, don't you think?" Trump said, complaining that Democrats "would rather see Trump do badly, ok, than our country do well. . . . It's very selfish."
Democrats reacted angrily to the president's characterization of "treason," a crime punishable by death.
"Whether I chose to applaud or not applaud is a First Amendment right that our forefathers wrote down and that generations before me have sacrificed many a life for," said Sen. Jon Tester (D-Mont.). "And I would just say the president's out of line with that statement."
In an unusual double-themed trip, first lady Melania Trump on Monday accompanied the president to Cincinnati, where she toured opioid addiction facilities at Cincinnati Children's Hospital.
President Trump, meanwhile, toured Sheffer Corp., a cylinder manufacturer outside Cincinnati, which gave its 126 employees $1,000 bonuses that management said were made possible by the package signed into law by Trump in December.
Sheffer CEO Jeff Norris, who led Trump on his tour, appeared earlier Monday on Fox News, taking a dig at House Democratic leader Nancy Pelosi's description of the tax-related bonuses as "crumbs."
"We consider that fine dining," Norris said.
Trump continued the ridicule, telling the Ohio crowd that Pelosi is "our secret weapon."
"She's a rich woman who lives in a beautiful house in California who wants to give all of your money away," Trump said to laughter.
Trump played up the tax cuts in his State of the Union address last week, claiming the package will have broad benefits, although it is weighted toward the wealthy and big corporations. February is when Trump has promised that Americans will begin seeing slightly larger paychecks because of the changes.
The Republican package passed in December as the most unpopular bill in decades, but a Monmouth University poll last week showed a significant swing toward the bill. Although 26 percent of Americans approved of the package in mid-December, support in the latest Monmouth poll rose to 44 percent.
Wagner reported and Erica Werner contributed to this report from Washington.