Senate Democrats on Monday refused to participate in a phone call with Supreme Court nominee Brett Kavanaugh over an allegation that he attempted to sexually assault a girl at a party during high school, and instead insisted that the FBI is needed to investigate the claim.

The Judiciary Committee’s top Democrat, Sen. Dianne Feinstein of California, said Democrats “will not participate” in the phone call with Kavanaugh that's scheduled with Senate Judiciary Committee staff at 5:30 p.m.

Instead, Feinstein called on the FBI to open an investigation into the claim by Christine Blasey Ford that Kavanaugh held her down at a party and tried to take her clothes off and assault her.

“In view of the enormity and seriousness of these allegations, a staff-only phone call behind closed doors is unacceptable and Democratic staff will not participate.” Feinstein said in a statement Monday afternoon. “This isn’t how things should be done and is in complete violation of how this committee has worked in the past."

[Durbin: Kavanaugh accuser needs a public hearing, not phone calls]

Feinstein said the FBI “has the resources and know-how to conduct an objective, independent evaluation of these sensitive allegations with appropriately trained investigators.”

Republicans and Democrats are battling over how to handle the last-minute claim, which surfaced in the media beginning late last week.

Judiciary Committee Chairman Chuck Grassley, R-Iowa, is so far refusing to hold a public hearing. Senate Majority Leader Mitch McConnell, R-Ky., backs Grassley’s decision.

Grassley has also scheduled a Thursday committee vote to advance Kavanaugh, but at least two panel Republicans say they want more information about the accusation.

Republican calls to slow down the process now threaten the GOP's plan to have the committee confirm Kavanaugh this week. As of late Monday, Republican leaders appeared to be content with allowing the committee to vote.

Kavanaugh told Senate Republicans that the accusations against him are a case of mistaken identity, and that he was never at a party like the one described by Ford.