The breathless text message from President Trump and his campaign landed soon after Speaker Nancy Pelosi announced a formal impeachment inquiry: “I need you on my Impeachment Defense Team.”

Within three hours of Ms. Pelosi’s announcement on Tuesday, Mr. Trump had raised $1 million, and his campaign manager said on Wednesday that Mr. Trump’s campaign and the Republican National Committee had raised $5 million in 24 hours. “Huge groundswell of support,” wrote Brad Parscale, Mr. Trump’s campaign manager, on Twitter.

The impeachment fight between the Democratic-led House of Representatives and the Trump White House may have initiated a constitutional clash, but for the political operatives involved in the 2020 campaign it also represents a potentially galvanizing moment to pry loose wallets.