U.S. Sen. Cory Booker on Wednesday called Attorney General Jeff Sessions' dismissal "an alarming development" and warned that curtailing the probe into possible collusion between President Donald Trump's campaign and Russian officials could lead to "a constitutional crisis."

The investigation by special counsel Robert Mueller has been overseen by Deputy Attorney General Rod Rosenstein, but the acting attorney general, Matthew Whitaker, could take over supervision and limit the scope of the probe.

Whitaker took over the Justice Department Wednesday as Trump requested and obtained the Sessions' resignation of Attorney General Jeff Sessions a day after the midterm elections.

"Jeff Sessions' firing at the hands of the president is an alarming development that brings us one step closer to a constitutional crisis," said Booker, D-N.J.

"I'm concerned that President Trump made this decision based on his fear of being implicated by Special Counsel Robert Mueller's investigation into Russian interference in the 2016 election and frustration with Sessions' recusal from that investigation."

Sessions had recused himself from the probe following revelations that he did have contacts with the Russian ambassador during the presidential campaign after saying during those confirmation hearings said he hadn't talked to any Russian official.

Booker, a member of the Senate Judiciary Committee and sponsor of a bipartisan bill making it harder to fire the special counsel, said Whitaker should also rescue himself "given his past comments targeting the Mueller investigation and advocating for limits on its scope and cuts to its funding."

The senator cited the ongoing probe when he opposed the nomination of Brett Kavanaugh to the U.S. Supreme Court, saying he could sit in judgment of the president who picked him.

Booker's legislation, sponsored with U.S. Sens. Lindsey Graham, R-S.C., Chris Coons, D-Del., and Thom Tillis, R-N.C., would allow Mueller to be fired only for good cause by a senior Justice Department official, require that the reasons be given in writing, and give him 10 days to appeal his dismissal to a federal judge.

While the Senate Judiciary Committee passed the measure in April, Senate Majority Leader Mitch McConnell, R-Ky., has refused to bring it up for a vote by the full Senate.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.