Sen. Claire McCaskill, facing a tough re-election battle in red state Missouri, said on Wednesday she will vote against confirming President Donald Trump's nominee to the U.S. Supreme Court, Brett Kavanaugh.

In a statement posted on Twitter, McCaskill said her decision is not based on recent allegations that Kavanaugh, currently a federal judge, assaulted a girl in high school, which Kavanaugh denies.

Instead, she said, she will vote against him because of "his positions on several key issues, most importantly the avalanche of dark, anonymous money that is crushing our democracy."

Trump, a Republican, won Missouri in the 2016 presidential election.