At a presser today, Reid told reporters he was taking another look at rules reform, but didn’t give a timeline. The senior leadership aide goes further, saying it’s hard to envision circumstances under which Reid doesn’t act.

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“Reid has become personally invested in the idea that Dems have no choice other than to change the rules if the Senate is going to remain a viable and functioning institution,” the aide says. That’s a long journey from where Reid was only 10 months ago, when he agreed to a toothless filibuster reform deal out of a real reluctance to change the rules by simple majority. Asked to explain the evolution, the aide said: “It’s been a long process. But this is the only thing we can do to keep the Senate performing its basic duties.”

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Asked if Reid would drop the threat to go nuclear if Republicans green-lighted one or two of Obama’s judicial nominations, the aide said: “I don’t think that’s going to fly.”

Reid has concluded Senate Republicans have no plausible way of retreating from the position they’ve adopted in this latest Senate rules standoff, the aide says. Republicans have argued that in pushing nominations, Obama is “packing” the court, and have insisted that Obama is trying to tilt the court’s ideological balance in a Democratic direction — which is to say that the Republican objection isn’t to the nominees Obama has chosen, but to the fact that he’s trying to nominate anyone at all.

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Reid believes that, having defined their position this way, Republicans have no plausible route out of the standoff other than total capitulation on the core principle they have articulated, which would be a “pretty dramatic reversal,” the aide continues.

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“They’ve boxed themselves in — their position allows them no leeway,” the aide says, in characterizing Reid’s thinking. “This is not a trumped up argument about the qualification of a nominee. They are saying, `we don’t want any nominees.'”

The aide says Reid believes he now has 51 Dem Senators behind a rules change, if it comes down to it. The Huffington Post reports that some Dem Senators who have previously opposed changing the rules — such as Barbara Boxer and Dianne Feinstein — are now open to it. “I believe that we are there,” the aide tells me.

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The key to understanding what’s happening now is that it is fundamentally different from the last “nuclear” standoff. In that one, Republicans blockaded a handful of executive nominations (such as Richard Cordray to head the Consumer Financial Protection Bureau) in part out of an objection to the function of the agencies they had been picked to lead. Republicans ultimately dropped their objections in exchange for Dems agreeing not to change the rules.

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