(CNN) The top Democrat on the Senate Judiciary Committee is accusing the panel's Republican chairman of not wanting to dig deeply into the Russia investigation, the latest sign of rising partisan tensions on Capitol Hill over President Donald Trump and Russia.

Sen. Dianne Feinstein of California has sent a slew of letters in recent weeks seeking documents and interviews from a host of key witnesses in the Trump orbit — and Senate Judiciary Chairman Chuck Grassley's has not signed on.

"We want him to sign on. I think there's an effort -- subtle -- not to go deeply," Feinstein said. "I hadn't realized it before. But I realize it now. And we're going to have to find a way to deal with it."

Grassley told CNN Tuesday that he's not stalling the committee's probe as Feinstein suggested, saying they were working to come to consensus on a range of issues.

"When she wants to send letters, she sends them to us, and if we want to sign on, we do that ... And she can decide to sign on the letters that we have," Grassley said. "And then the bottom line of it is that if people come up here as a result of her letters or my letters to be questioned, then her staff and my staff can sit in on each other's and ask questions. So at the level of actually asking questions, there's a great deal of cooperation."

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