Washington — Republican Senator Susan Collins announced Tuesday she will vote to acquit President Trump in his Senate trial, telling CBS News she believes the president has learned a "pretty big lesson" from impeachment and will be "much more cautious" about seeking foreign assistance in the future.

"I believe that the president has learned from this case," Collins said in an exclusive interview with "CBS Evening News" anchor Norah O'Donnell on Tuesday, before a speech on the Senate floor about her decision. "The president has been impeached. That's a pretty big lesson."

The Senate plans to vote Wednesday on whether to convict Mr. Trump on two articles of impeachment: abuse of power and obstruction of Congress. Collins and Senator Mitt Romney were the only two Republicans who voted last week to allow witnesses in the impeachment trial, an effort that ultimately failed.

Despite her support for more witnesses, Collins said she would vote to acquit Mr. Trump of the charges, arguing that his actions don't amount to high crimes and misdemeanors.

"I'm voting to acquit. Because I do not believe that the behavior alleged reaches the high bar in the Constitution for overturning an election, and removing a duly elected president," Collins said.

The Maine Republican told CBS News she believes the bipartisan condemnation of the president's efforts to pressure Ukraine will give him pause about foreign assistance moving forward.

"He was impeached. And there has been criticism by both Republican and Democratic senators of his call," Collins explained, referring to his July 25 call with the Ukrainian President Volodymyr Zelensky, in which Mr. Trump asked for an investigation into former Vice President Joe Biden. "I believe that he will be much more cautious in the future."

Collins said the July conversation with was "far from a perfect call," rejecting the president's repeated characterization of it.

MORE: In an exclusive interview with @NorahODonnell, Sen. Susan Collins (R-ME) says Pres. Trump's July 25 call with Ukrainian Pres. Volodymyr Zelensky was "improper" and "far from perfect." "He should not have mentioned Joe Biden in it," Collins says. https://t.co/WqaRWXlmaD pic.twitter.com/LqA1ck3oLv — CBS Evening News (@CBSEveningNews) February 4, 2020

"The president's call was wrong. He should not have mentioned Joe Biden in it, despite his overall concern about corruption in Ukraine," Collins said. "The president of the United States should not be asking a foreign country to investigate a political rival. That is just improper. It was far from a perfect call."

Collins, who is up for reelection this year, said her campaign prospects weren't a factor in her decision to vote to acquit.

"I'm sure there are going to be people unhappy with me in Maine. All I can do is apply the constitutional standard. And that's my job," Collins said. "My job is not to weigh the political consequences, but to do impartial justice to live up to the oath that I took."