The representative from Colorado, first elected in 1996, spent 14 years as the Democrats’ chief deputy whip and often holds the gavel in the House.

WASHINGTON — When Speaker Nancy Pelosi was looking for someone to preside over the historic debate over impeaching President Trump at the end of 2019, she chose a veteran Democrat who had impressed her with a tough, skillful parliamentary hand: Representative Diana DeGette of Colorado.

“Sitting here in the speaker’s chair, all I can think is how serious this debate is for the future of our republic,” she wrote on Twitter at the time. “The fact that I’ve been asked to preside over the House for this important moment in our nation’s history is truly an honor.”

Now Ms. Pelosi has turned to Ms. DeGette again, this time as an impeachment manager to prosecute the case against Mr. Trump in the Senate. In picking the Colorado congresswoman, she chose someone with deep experience in the House and in the presiding officer’s chair.

Ms. DeGette, first elected in 1996, spent 14 years as the Democrats’ chief deputy whip — the member of leadership responsible for counting votes, known as whipping in congressional parlance. She often holds the gavel in the House, rotating in and out of the chair as members customarily do.

On Capitol Hill, she has carved out a niche in health policy, and as a champion of reproductive rights — a legislative portfolio that dates to her days as a state legislator in the 1990s, when she wrote the so-called “bubble bill” creating an eight-foot privacy bubble around any person within 100 feet of a Colorado health facility, including abortion clinics. The bill survived a Supreme Court challenge.

She is also the author of the 21st Century Cures Act, a 2016 measure intended to help accelerate medical product development and bring new innovations and advances to patients who need them faster and more efficiently. It was among the last bills signed into law by President Barack Obama.

When Democrats reclaimed the House majority in 2018, Ms. DeGette announced her intention to run for the top whip’s slot, which would have made her the No. 3 Democrat in the House. But she ultimately withdrew from the race, citing “internal pressure” from Democrats to align behind the existing leadership triumvirate of Ms. Pelosi; Representative Steny H. Hoyer of Maryland, the majority leader; and Representative James E. Clyburn of South Carolina, the current whip.

On Tuesday, she said she was “honored” to help with this second impeachment effort.

“Trump has shown he is a real danger to this country,” she wrote on Twitter. “I look forward to doing my part to remove him from office immediately.”