(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.
Senate Majority Leader Mitch McConnell of Kentucky has vowed not to put the bill on the floor, although Grassley said that would not affect his decision to take the bill up with his committee.
Feinstein declined to detail her specific concerns Tuesday, but she has previously raised questions about the Iowa Republican's proposal to require the special counsel to report to Congress prosecuting decisions in special counsel investigations.
"In the name of transparency we should not create new reporting requirements that could bring about obstruction or political pressure," Feinstein said at last week's committee meeting.
Grassley has circulated a draft of the amendment to committee members but has not released it publicly. Grassley himself has said he has his own concerns about the bill's constitutionality, but that his amendment was intended as a supplement to improve the bill. The amendment would require the special counsel to inform Congress about changes to the scope of the probe in addition to explaining prosecutorial decisions, as well as provide Congress with advance notice of the dismissal of a special counsel.
Feinstein says she is hopeful the amendment will be changed before the markup on Thursday.
On Tuesday afternoon, Grassley said he and Feinstein had spoken earlier in the day and she did not raise the matter then.
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