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Sen. Heidi Heitkamp, D-N.D., speaks during the Tribal Unity Impact Days hosted by the National Congress of American Indians in Dirksen Building on September 12, 2018.
A spokeswoman for Heitkamp's campaign confirmed the embattled senator's position on the high court nominee in a phone call with CNBC. Heitkamp first announced her position on the vote in an interview with local media outlet WDAY on Thursday.
"If this were a political decision for me I certainly would be deciding the other way," Heitkamp said in the interview. "History will judge you, but most importantly you will judge yourself."
In a statement released shortly after, Heitkamp criticized lawmakers for their handling of Kavanaugh's nomination.
"Both sides horribly handled the process around this nomination," Heitkamp said in the statement. "We must learn from these mistakes."
She also referenced her past support for President Donald Trump's prior Supreme Court pick, Neil Gorsuch, explaining that she "voted for Justice Gorsuch because I felt his legal ability and temperament qualified him to serve on the Supreme Court."
She added: "Judge Kavanaugh is different."
Kavanaugh's demeanor was strongly questioned by his opponents after he testified before the Senate Judiciary Committee last week, when the visibly furious conservative judge lashed out at Democrats and political opponents.
Heitkamp, facing re-election in a state Trump won by nearly 40 percentage points in 2016, is considered the most endangered Democrat in the Senate.
As the November midterm elections approach, Heitkamp's poll numbers have begun to plummet. A Fox News poll published Wednesday showed Heitkamp 12 percentage points behind her challenger, Republican Rep. Kevin Cramer.
Cramer, with support from national Republican groups, had hammered the incumbent Democrat over her indecision on Kavanaugh.
"I'm deeply disappointed with Senator Heitkamp's decision to vote no on Judge Kavanaugh's nomination to the Supreme Court. Clearly, she has decided to vote with Chuck Schumer, and not the people of North Dakota," Cramer said in a statement.
In a separate interview with CNBC, Cramer said he was surprised by Heitkamp's move. But he insisted that her decision does not necessarily mean she has given up on her re-election fight.
"I'm probably the most surprised person in the country. I had her pegged as a yes from the very beginning for a very long time," Cramer said. "But I don't see her waving the white flag. I don't, because this is a woman who has a ton of money — and I don't — with a month left in the campaign."
Heitkamp has raised just over $10 million throughout the 2018 election cycle while Cramer has brought in $3.2 million, according to the Center for Responsive Politics.
The senator has repeatedly attacked Cramer over comments he made about Christine Blasey Ford's sexual assault accusation against Kavanaugh. In an interview last month, he said "nothing evidently happened" between the judge and Ford. He later said that he meant "there was no type of intercourse or anything like that."
Kavanaugh vehemently denied the allegations by Ford and other women in testimony before the Senate Judiciary Committee last week, saying, "I've never sexually assaulted anyone."
West Virginia's Sen. Joe Manchin, another Democrat up for re-election in a state Trump won, has yet to weigh in on how he will vote.
Meanwhile, undecided moderate Republican Sens. Susan Collins of Maine and Lisa Murkowski of Alaska are under the most scrutiny. Republicans, who have a 51-49 majority, can only afford to lose one vote. A tie would be broken by Vice President Mike Pence.