Colorado’s U.S. Rep. Diana DeGette wielded the gavel as the U.S. House of Representatives took up debate Wednesday to impeach President Donald Trump on charges he abused his power and obstructed Congress.

The Denver Democrat was chosen by Speaker Nancy Pelosi to preside as speaker pro tempore for what promised to be a long day of speeches and procedural wrangling before an expected vote to send two article of impeachment to the Senate.

“This is a sad and somber moment in our nation’s history, and the responsibility to preside is something I won’t take lightly,” DeGette tweeted Wednesday morning.

She added in a statement: “None of us came to Congress to impeach a president, but every one of us — when we assumed office — took an oath to uphold the Constitution.”

DeGette, a former chief deputy whip for the House Democrats, was tapped to conduct the historic session because she knows the chamber’s often arcane rules inside and out and has a reputation as her caucus’s most capable speaker pro tem, a senior congressional aide said.

“Her colleagues trust her ability to keep control when proceedings threaten to go off the rails,” the aide told Colorado Politics.

Before the House began the planned six hours of debate over the measures — divided equally, with three hours total allotted to the Democrats and the same to the GOP — Republican lawmakers called for procedural votes to delay or derail the proceeding.

Over the course of nearly three hours, the majority Democrats voted along party line to defeat a motion to adjourn and another to condemn the Democratic committee leaders for the way they handled hearings investigating the president’s conduct.

“We deserve better than the flawed process that led to this flawed outcome,” said U.S. Rep. Tom Cole, R-Oklahoma, the ranking member of the House Rules Committee, before the formal debate commenced.

The vote to approve the rules for the proceedings passed overwhelmingly, 228-197, with two Democrats voting no and a yes vote from conservative U.S. Rep. Justin Amash, an independent who left the Republican Party earlier this year over his support for impeachment.

Trump denounced the proceedings in a tweet: “SUCH ATROCIOUS LIES BY THE RADICAL LEFT, DO NOTHING DEMOCRATS. THIS IS AN ASSAULT ON AMERICA, AND AN ASSAULT ON THE REPUBLICAN PARTY!!!!”

Opening the debate for the Democrats, Pelosi argued the contrary position.

“It is a matter of fact that the president is an ongoing threat to our national security and the integrity of our elections, the basis of our democracy,” she said. “If we do not act now, we would be derelict in our duty. It is tragic that the president’s reckless actions make impeachment necessary. He gave us no choice.”

As the debate commenced, DeGette briefly departed the speaker’s rostrum to take a break, a spokeswoman said, but would be presiding through the day. 

A former state lawmaker, DeGette, 62, was first elected to represent the Denver-based 1st Congressional District in 1996 and has been re-elected to the heavily Democratic seat 11 times.

She’s the only member of Colorado’s congressional delegation who was in office the last time a president was impeached, in 1998 and 1999, when Bill Clinton was charged by House Republicans with lying under oath and obstruction of justice. During the lame duck session at the end of her first term, DeGette voted against the articles of impeachment in the full House, and the Senate later acquitted Clinton.

This is a developing story. Check back for more.

The Associated Press contributed to this report.