Rep. Diana Degette, member presiding over the U.S. House of Representatives, pounds the gavel to open the session to discuss…
Rep. Diana Degette pounds the gavel to open the US House of Representatives session to discuss rules ahead a vote on two articles of impeachment against President Donald Trump, Dec. 17, 2019.

The U.S. House of Representatives headed Wednesday toward impeaching President Donald Trump, accusing him of abusing the power of the presidency to benefit himself politically and then obstructing Congress as it investigated his actions.

House Speaker Nancy Pelosi opened hours of debate,  telling lawmakers that Trump, by pressing Ukraine to investigate one of his chief 2020 Democratic rivals, former Vice President Joe Biden, and then standing in the way of a congressional investigation, "gave us no choice" but to pursue his impeachment.

"If we do not act," she said, "we would be derelict in our duty. Today we are here to defend democracy."

House Speaker Nancy Pelosi speaks as the House of Representatives debates the articles of impeachment against President Donald Trump, at the Capitol in Washington, Dec. 18, 2019.

Republican Congressman Doug Collins, one of Trump's staunchest supporters, dismissed Pelosi's assessment of Trump, contending Democrats have wanted to impeach Trump since he was elected three years ago. Now, Collins said, rather than the House impeaching Trump less than a year before he seeks re-election in 2020, it should be "a matter for the voters" to decide his fate.

"The president did nothing wrong," Collins said, in pushing Ukrainian President Volodymyr Zelenskiy to open the Biden investigation at the same time Trump was withholding $391 million in military aid Kyiv wanted to help pro-Russian separatists in eastern Ukraine.

The Georgia congressman rejected Democratic claims that Trump engaged in a reciprocal quid pro quo -- blocking the military assistance until he got the Biden investigation -- because "nothing was ever done to get the money," with Trump dispatching the aid in September without Zelenskiy launching a Biden probe.

The House planned six hours of debate before voting on the two articles of impeachment. One Democratic lawmaker after another stated the case for impeachment alternated with Republicans decrying the allegations as unfounded and unwarranted, the product of an unfair and rushed investigation led by the House's Democratic majority.

But with Democrats firmly in control of the chamber, the outcome does not appear in doubt: the Republican Trump would become the third U.S. president to be impeached in the country's 243-year history.

Trump would then face a trial in the Republican-majority Senate next month, although his conviction and removal from office remains highly unlikely. The trial would occur 10 months before Trump is running for re-election to a second term in next November's national election.

Trump, hours after he sent a scathing six-page letter to Pelosi deriding Democrats' impeachment effort against him, took to Twitter early Wednesday to continue to attack his political opponents.

"Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG!" he said. "A terrible Thing. Read the Transcripts. This should never happen to another President again. Say a PRAYER!"

The impeachment votes are scheduled about the same time Trump is set to speak at a campaign rally in the midwestern state of Michigan, one of the pivotal states he won in the 2016 election that made him the 45th U.S. president.

In one article of impeachment, Democrats leading the charge against Trump accuse him of abusing the presidency by orchestrating a months-long effort to get Zelenskiy to investigate Biden, his son Hunter Biden's work for a Ukrainian natural gas company and a debunked theory that Ukraine meddled in the 2016 U.S. election to undermine Trump's candidacy, with the military aid hanging in the balance at the same time. 

The second accusation alleges that Trump obstructed Congress by refusing to turn over thousands of pages of Ukraine-related documents to impeachment investigators and blocking key aides from testifying at weeks of inquiries Democrat-controlled House committees conducted about Trump's actions.

House Intelligence Committee Chairman Rep. Adam Schiff
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The nation's 45th president is on track to become only the third commander in chief to be impeached

Trump's Republican supporters have steadfastly defended him and no Republican has announced their intention to vote for his impeachment. They contend, often like Collins, that Trump did nothing wrong, ultimately releasing the military assistance in September without Zelenskiy launching any investigations of the Bidens, proof that Trump did not engage in a reciprocal quid pro quo deal with the Ukrainian leader.

Trump has on countless occasions has described his late July call with Zelenskiy as "perfect," when he asked him to "do us a favor," to investigate the Bidens and Ukraine's purported role in the 2016 election. As the impeachment controversy mounted, Trump has subsequently claimed the "us" in his request to Zelenskiy referred not to him personally but to the United States.

WATCH: Trump faces possible impeachment 

In his ranting letter to Pelosi, Trump accused Democrats of engaging in a “perversion of justice” and an “attempted coup,” claiming his opponents had declared "open war on American Democracy."

Among other vitriolic adjectives, Trump called the impeachment process “invalid,” “spiteful,” “meritless,” “disingenuous,” “baseless,” and  “preposterous.”

Pelosi called the letter "really sick."

She wrote to House members, saying that in considering whether to impeach a president, they would be exercising "one of the most solemn powers granted to us by the Constitution."

Pelosi, for months reluctant to pursue Trump's impeachment, concluded, “Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress as he demanded that he is above accountability, above the Constitution and above the American people. In America, no one is above the law."