Collins, R-Maine, had been considered a potential swing vote for Democrats after siding with them in favor of allowing witnesses in the trial but said she would vote to acquit Trump's on both articles of impeachment.
She acknowledged on the Senate floor that Trump's actions were improper and "demonstrated very poor judgment" but ultimately decided they did not warrant removal from office.
"Except when extraordinary circumstances require a different result, we should entrust to the people the most fundamental decision of a democracy, namely who should lead their country," she said.
Collins' comments come after Democratic House managers made final arguments Monday for removing Trump, urging senators to show a U.S. president is not above the law. Both sides were given 2 hours to make their closing arguments.
The Senate also voted last week not to hear from any witnesses or introduce new evidence.
Senate Republican leader Mitch McConnell urged the Senate to "do what we were created to do" and vote to acquit Trump, calling the impeachment of Trump abuse of the House's power.
"If Washington Democrats have a case to make against the president's re-election, they should make it," he said. "They don't get to rip away the choice from voters because they're afraid they might lose again."
"The Constitution draws a line that is much clearer than the president's lawyers have tried to argue," Warner said. "The president crossed it. He abused his power."
On Tuesday, Sen. Elizabeth Warren, D-Mass., said she anticipates the Republican Senate will vote to acquit Trump and said she would vote in favor of censuring him.
"Let's be clear about where we stand right now. The Republicans in Senate have locked arms to protect an out-of-control president," Warren said.
Sen. Rand Paul, R-Ky., read the name of the alleged whistle-blower who initially reported on Trump's conduct toward Ukraine aloud on the Senate floor Tuesday after Chief Justice John Roberts, who presided over the impeachment trial, declined to read a question naming the person.
"They made a big mistake not allowing my question. My question did not talk about anybody who is a whistle-blower, my question did not accuse anybody of being a whistle-blower, it did not make a statement believing someone was a whistleblower," Paul said. "I simply named two people's names because I think it's very important to know what happened."
Paul added that he supports protections against reprisal for whistle-blowers, but does not believe they should remain anonymous.
"In the first month of office, in January of 2017, they were already plotting the impeachment," he said. "And you say 'Well, we should protect the whistle-blower and the whistle-blower deserves anonymity.' The law does not preserve anonymity. His boss is not supposed to say anything about him, he's not supposed to be fired. I'm for that."