WASHINGTON — House Democrats, confident that public support is growing for their impeachment inquiry as they head into a second week of nationally televised hearings, are sharpening their tone as they make their case that President Trump withheld military aid from Ukraine in a bid to force its leader to investigate his political rivals.

Quid pro quo is out. Extortion and bribery are in.

The shift was inaugurated last week by Speaker Nancy Pelosi, when she said the “devastating testimony” delivered by top diplomats “corroborated evidence of bribery.” On Monday, the speaker kicked it up a notch, using the word “extortion” to address Republican claims that there was no wrongdoing because Mr. Trump eventually released the aid.

“The fact is, the aid was only released after the whistle-blower exposed the truth of the President’s extortion and bribery,” she wrote in a letter to her Democratic colleagues, “and the House launched a formal investigation.”

As lawmakers plunge into a jam-packed hearing schedule with nine witnesses lined up this week, Ms. Pelosi is clearly trying to tighten the screws on the president — and target her message to skeptical voters — with words that evoke criminal wrongdoing.