Maine Republican Sen. Susan Collins, a key swing vote in President Trump's impeachment trial, announced Tuesday she would vote to acquit on both articles of impeachment -- noting that Democrats' "abuse of power" charge "did not even attempt" to allege that Trump had committed a crime, and instead constituted a "difficult-to-define, non-criminal act."

Even as she criticized Trump's behavior as "flawed," Collins further slammed House Democrats for delaying transmitting the articles of impeachment to the Senate for more than a month, saying the stalling and posturing undercut their arguments that the president was an imminent threat.

Last week, along with Utah GOP Sen. Mitt Romney, Collins had broken ranks with her fellow Republicans to vote in favor of additional witnesses in the Senate trial. Trump, who is now set to be overwhelmingly acquitted by the Senate on Wednesday, will deliver the annual State of the Union address before Congress Tuesday night amid record-high approval ratings.

Collins began her remarks on the Senate floor by observing the increasingly partisan nature of the impeachment process.

"For more than 200 years after our Constitution was adopted, only one president faced an impeachment trial before the United States Senate -- that was Andrew Johnson in 1868," she said. "But now, we are concluding our second impeachment trial in just 21 years."

"We are concluding our second impeachment trial in just 21 years." — Maine GOP Sen. Susan Collins

Pointing to her vote to acquit President Bill Clinton during his impeachment trial in 1999, Collins said she has consistently believed that wrongdoing in office, even if proven, is not necessarily impeachable conduct.

"In the trial of President Clinton, I argued that in order to convict, 'We must conclude from the evidence presented to us with no room for doubt that our Constitution will be injured and our democracy suffer should the president remain in office one moment more,'" Collins said. "The House Managers adopted a similar threshold when they argued that President Trump’s conduct is so dangerous that he 'must not remain in power one moment longer.'"

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"In its first Article of Impeachment against President Trump, the House asserts that the President abused the power of his presidency," Collins said. "While there are gaps in the record, some key facts are not disputed. It is clear from the July 25, 2019, phone call between President Trump and Ukrainian President Zelensky that the investigation into the Bidens’ activities requested by President Trump was improper and demonstrated very poor judgment."

However, Collins asserted, there was "conflicting evidence in the record" concerning Trump's motivation for the request.

"The House Managers stated repeatedly that President Trump’s actions were motivated 'solely' for his own political gain in the 2020 campaign, yet the president’s attorneys argued that the President had sound public policy motivations, including a concern about widespread corruption in Ukraine," Collins said. "Regardless, it was wrong for President Trump to mention former Vice President Biden on that phone call, and it was wrong for him to ask a foreign country to investigate a political rival."

Trump's conduct, though, did not constitute an impeachable "abuse of power" as Democrats had attempted to define the term, Collins argued.

"The House Judiciary Committee identified in its report crimes that it believed the president committed," Collins said. "Article I, however, does not even attempt to assert that the President committed a crime. I sought to reconcile this contradiction between the report and the Articles in a question I posed to the House Managers, but they failed to address that point in their response.

"While I do not believe that the conviction of a President requires a criminal act, the high bar for removal from office is perhaps even higher when the impeachment is for a difficult-to-define noncriminal act," Collins said. "In any event, the House did little to support its assertion in Article I that the president 'will remain a threat to national security and the Constitution if allowed to remain in office.'"

Therefore, Collins concluded, as with the Clinton trial, she would vote to acquit.

"I do not believe that the House has met its burden of showing that the president’s conduct – however flawed – warrants the extreme step of immediate removal from office," Collins said. Nor does the record support the assertion by the House Managers that the President must not remain in office one moment longer. The fact that the House delayed transmitting the Articles of Impeachment to the Senate for 33 days undercuts this argument. For all of the reasons I have discussed, I will vote to acquit on Article I."

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Concerning the "obstruction of Congress" charge against Trump, Collins began by characterizing the allegation as a "dispute over witnesses and documents between the legislative and executive branches." As a general principle, Collins said, "an objection or privilege asserted by one party cannot be deemed invalid – let alone impeachable – simply because the opposing party disagrees with it."

"Before the House even authorized its impeachment inquiry, it issued 23 subpoenas to current and former administration officials," Collins said. "When the House and the President could not reach an accommodation, the House failed to compel testimony and document production. The House actually withdrew a subpoena seeking testimony from Dr. Charles Kupperman, a national security aide, once he went to court for guidance. And the House chose not to issue a subpoena to John Bolton, the National Security Advisor whom the House has identified as the key witness."

Collins went on: "At a minimum, the House should have pursued the full extent of its own remedies before bringing impeachment charges, including by seeking the assistance of a neutral third party – the Judicial Branch. In making these choices, the House substituted its own political preference for speed over finality. The House Managers described impeachment as a “last resort” for the Congress. In this case, however, the House chose to skip the basic steps of judicial adjudication and instead leapt straight to impeachment as the first resort.

"Therefore, I will vote to acquit on Article II," she said.

Collins' remarks came a day after West Virginia Democratic Sen. Joe Manchin, a moderate in a pro-Trump state, strongly urged the Senate to consider censuring rather than voting to convict and remove the president.

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"Never before in the history of our Republic has there been a purely partisan impeachment vote of a president," Manchin said. "Removing this president at this time would not only further divide our deeply divided nation, but also further poison our already-toxic political atmosphere."

But he also strongly condemned Trump: "The president asked a foreign government to intervene in our upcoming election," Manchin said. "He defied lawful subpoenas from the House of Representatives."

Alaska Republican Sen. Lisa Murkowski, another closely watched swing vote, announced on the Senate floor Monday night that she "cannot vote to convict" Trump.

Murkowski, whose comments closed out a day of debate on the floor over the articles of impeachment, said the "Constitution provides for impeachment but does not demand it in all instances." While most Republican senators are expected to vote to acquit Trump, Murkowski had been considered a possible vote against the president.

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In her floor speech, she said Trump's "behavior was shameful and wrong" with Ukraine but argued against removing him from office, calling for voters to make a judgment in November's election.

"The response to the president's behavior is not to disenfranchise nearly 63 million Americans and remove him from the ballot," she said. "The House could have pursued censure and not immediately jumped to the remedy of last resort."

The Alaska senator added: "The voters will pronounce a verdict in nine months and we must trust their judgment."

Fox News' Andrew O'Reilly contributed to this report.