Can Charles Schumer ever be master of the Senate? It’s hard to imagine on the basis of the lackluster nature of his opposition to President Obama’s nuclear-arms deal with the regime in Iran.

“Master of the Senate” is the title of the volume of Robert Caro’s biography of Lyndon Baines Johnson that covers LBJ’s leadership in the upper house of Congress. It includes his maneuvering to pass the 1957 Civil Rights Bill.

What a ruthless, manipulative, bombastic, cunning leader LBJ was. He twisted arms, threatened, cajoled, pontificated and sulked — and outworked his colleagues, too — to get his way in the world’s greatest deliberative body.

Particularly on a matter of conscience, like civil rights. Can anyone imagine LBJ taking the kind of powder that Mr. Schumer is taking now on Mr. Obama’s plan to appease Iran?

LBJ, of course, didn’t face the kind of gutter attacks that Mr. Schumer is facing. The New York Post, in an important editorial on Monday, reported that “anti-Semitism is all over the drive to make Chuck Schumer shut up about his opposition to the nuke deal.”

The administration has been pulling out all the stops. Yet with every passing day, what I at first called “Schumer’s finest hour” is turning into a shocking collapse, as the senator privately shrinks from the fight.

This is being reported in a dispatch of the Washington Web site Politico. Its headline is “Chuck Schumer working the phones on Iran.” It turns out he’s furiously dialing his colleagues to assure them he will, as Politico puts it, “not be whipping opposition to the deal.”

Why not? Schumer himself has warned that if, after 10 years, Iran is the same nation that it is now, we’ll be worse off with the agreement than we would be without it. Given how close the question is in the Senate, leadership is particularly urgent.

The first test is due in September, when the Senate will voice its disapproval. The betting — but not the certainty — is that there will be at least 60 senators prepared to vote against the deal. That will enable the question to be put to a vote.

A vote of disapproval in the Senate, incidentally, would be a huge setback for a deal of this importance. It would put into sharp relief the fact that Mr. Obama is doing a deal with the ayatollahs without the support of even a majority of the Senate, never mind the two-thirds required for a treaty.

Obama has already said he will veto such a rejection. So the question is whether enough Democrats can be rallied to override the president’s veto. That would require a vote of at least 67 senators.

At the moment, Politico reports, the question is “too close to call.” It counts only 17 of the 44 Democrats in the Senate as indicating they will vote with the president. Another dozen, Politico says, are leaning toward voting “yes” on the deal.

If ever a situation were made for LBJ-type senatorial leadership, this is it. Mr. Caro makes this clear in his account of how LBJ led the Senate to passage of the 1957 Voting Rights Act.

Similarities exist between LBJ’s predicament and Schumer’s. Johnson was caught between his southern loyalties and his desire to be a national leader, Mr. Caro notes. Schumer is torn between his pro-Israel sentiments and his party ambitions.

As the showdown was looming in the Senate, LBJ spent several weeks back home in Texas, playing dominoes with family members and cronies. Mr. Schumer has been back in New York, working local issues.

Once in action, though, Johnson wasn’t shy about threatening even the president.

At one point he went secretly to tell President Dwight Eisenhower that unless he watered down the Civil Rights Bill, Johnson would scuttle it — and the rest of Ike’s program.

The civil-rights measure eventually passed with 43 Republicans and 29 Democrats voting “aye” and no Republicans and 18 Democrats voting “nay.”

Is Schumer up to that kind of fight with Obama so as to get the president to go back to the table with the ayatollahs? According to Politico, some of Schumer’s conversations seem designed to shore up support for his own ambitions.

It quotes such on-the-fence senators as Sheldon Whitehouse (D-RI), Claire McCaskill (D-Mo.), and Joe Manchin (D-W.Va.) as assuring Schumer they’d back him for leader. It quotes Schumer as assuring them he wouldn’t twist their arms.

Is it possible to imagine Lyndon Johnson in such a situation? Particularly when it was a matter of high principle — like both the Civil Rights Act and opposition to the Iran deal — LBJ would have been master of the situation.

This column originally appeared in the New York Post.