Why now? Why did Senate Majority Leader Harry Reid (D) of Nevada choose a cool, fall day just before the Senate’s Thanksgiving recess to drop the “nuclear option” on this chamber and end the right of a minority of senators to block, or filibuster, most presidential nominations?

First, let’s clear up a matter of nomenclature – and meaning. Senator Reid’s move may be referred to as the “nuclear option,” but it’s really more like a conventional bomb. Going nuclear would have meant completely destroying the filibuster. What the Democrats did today was knock out only part of it.

From now on, it will only take a majority vote, instead of the required (and difficult to muster) 60 votes to overcome a filibuster that’s blocking a presidential nominee from Senate approval. That change, though, does not apply to Supreme Court nominees or to legislation – some of the most important and visible business of the Senate.

The reasons for the move Thursday vary, ranging from what Senate leaders are saying to what they're not saying.

Start with the official explanation. Reid, in laying out his reasoning for ending a defining element of Senate culture, said it was time to fix a Senate that is “broken.” He blamed accelerated Republican obstruction, citing 168 filibusters of executive and judicial nominations since the founding of the Republic – half of them during the Obama administration.

Republicans dispute the count, saying that Reid jumps to declare a filibuster even before there's any public GOP opposition to a nominee. Still, there's no question, he’s right about the trend, though gridlock has many fathers.

Both parties have expressed frustration with the filibuster over the years, but the straw that broke the camel’s back appears to have been the recent Republican blocking of three judicial nominees to the D.C. Circuit Court. That court is considered by legal experts to be second in importance only to the Supreme Court, because it rules on disputes over actions by the White House and its federal agencies, including federal regulations and disputes over executive power.

But it's especially important to Democrats, who are eager to fill judicial vacancies on an appeals court that is of fundamental import to the White House. When minority leader Mitch McConnell (R) of Kentucky, responding to Reid's move, said that the president’s agenda “runs through” the bureaucracy and the D.C. Circuit Court, and that the court is important because the administration wants to pursue “an aggressive regulatory agenda,” he was right. The court has become increasingly important to the president, as Congress, notably the GOP-controlled House, blocks his legislative initiatives.

Particularly noteworthy about the timing of Reid’s "November surprise" is the obvious political need to change the subject away from the disastrous rollout of Obamacare, toute suite. Obamacare is toxic to the Democrats right now, with President Obama’s approval rating sinking by 9 points since October, to a record low 37 percent, according to a CBS News poll. Senator McConnell pointed this out, of course, but at the same time, Republicans have been doing everything they can to keep those approval numbers heading south.

While most Democrats strongly opposed a proposed GOP "nuclear option" to protect President George W. Bush's judicial nominees when Republicans controlled the Senate – a bipartisan coalition of senators eventually killed it – the fallout is now politically acceptable to Democrats, who passed the rule change without GOP support. Three Democrats did not back Reid: Sens. Carl Levin of Michigan, Joe Manchin of West Virginia, and Mark Pryor of Arkansas.

Even though McConnell told reporters he’s not talking reprisal now, Senator Pryor, a member of the so-called Gang of 14, is concerned now, as he was in 2005, about how the "nuclear option" will affect the bipartisanship that still exists in the Senate.

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“I’ve played key roles in … bipartisan coalitions to help us reach common-sense solutions that both sides of the aisle can support," he said in a statement. "This institution was designed to protect – not stamp out – the voices of the minority.”

Unilaterally removing that protection, even in this limited way, is bound to invite retaliation. But apparently, Reid doesn't fear it. Says David Hopkins, a political scientist at Boston College: “The Democrats have decided things can’t get much worse than they already are."