WASHINGTON — Democratic efforts to highlight sexual assault charges that are more than 30 years old have been dismissed by supporters of Judge Brett M. Kavanaugh as the dredgings of ancient history. But the judge’s response to those accusations has raised new issues that go to the core of who President Trump’s Supreme Court nominee is right now: his truthfulness, his partisanship and his temperament.

For Democrats determined to derail Judge Kavanaugh, his performance last week before the Senate Judiciary Committee — his dissembling about his teenage years; his playing down drinking in high school and college; his raw, angry emotions; and his broadsides against Democratic questioners — is proving to be a new avenue of attack, if the accusations of sexual assault are not enough to swing the votes of three key Republicans and two undecided Democrats.

“The issues of credibility and temperament are not something that happened 30 years ago; they’re about Judge Kavanaugh today and how he is as a 53-year-old,” Senator Chuck Schumer of New York, the Democratic leader, said in an interview on Monday, giving a hint of the Democrats’ strategy. “I think there are serious questions about both his credibility and his temperament that may, to some senators, be more important than the activities that occurred in high school.”

As the investigation of past actions continues, lawmakers in both parties are parsing Judge Kavanaugh’s testimony from last week. At least one undecided Republican, Senator Jeff Flake of Arizona, took issue on Monday with the nominee’s angry treatment of Democrats who questioned him about his drinking in high school. Judge Kavanaugh parried a question from Senator Amy Klobuchar, Democrat of Minnesota, about whether he had ever blacked out from drinking — not by answering the question, but by asking the senator if she had ever blacked out. He later apologized.