(CNN) Sen. Dianne Feinstein of California, the top Democrat on the Senate Judiciary Committee, said Tuesday she will oppose legislation to protect special counsel Robert Mueller if an amendment from Judiciary Chairman Chuck Grassley is added to the bill.

Feinstein told CNN she has three concerns about the amendment Grassley plans to propose at Thursday's committee markup, and that she would oppose the bill "in its entirety" if the amendment is tacked on.

"There are three specific portions that I have problems with, and I believe others will have problems with," Feinstein said.

Feinstein's potential opposition, if other Democrats join with her, could sink the bill in the committee, as several Republicans are opposed to the measure on constitutional grounds.

The special counsel bill took on a renewed urgency earlier this month amid Democratic concerns that President Donald Trump might try to fire Mueller, although Republicans say that's not their impetus to move forward with the legislation. The bipartisan bill, which is co-sponsored by Democratic Sens. Chris Coons of Delaware and Cory Booker of New Jersey and Republican Sens. Thom Tillis of North Carolina and Lindsey Graham of South Carolina, would give Mueller and other special counsels the ability to seek a 10-day expedited judicial review of their firing.

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