So here’s today’s update in the slow, inexorable march towards filibuster reform: Democrats took yet another step in the direction of changing the rules by a simple majority vote — i.e., exercising the dreaded “nuclear option.”

For the second straight day, Harry Reid and Mitch McConnell traded sharp barbs on the Senate floor over what now looks to be inevitable. The key to today’s exchange was that it focused specifically on the nuclear option. Reid lambasted McConnell for a newly unearthed quote from 2005 in which McConnell endorsed — in principle — the right to change the rules by a simple majority vote; McConnell hit back by saying Republicans ultimately decided against taking the plunge.

The tit-for-tat aside, what really matters is that Reid stepped way out on a limb in the direction of exercising that “nuclear option.”

So here’s what is likely to happen, according to a senior Senate Democratic leadership aide. Dems will likely pass reforms that include ending the filibuster on the motion to proceed and on on motions to move to conference; and forcing “talking filibusters,” which would require a much more public role for filibustering.

Dems may not change the rules on the first day of the session. Rather, the aide says, they are likely to do a rules change, almost certainly in January, via what’s known as overruling the chair. Democrats ask the chair for a ruling on whether it is within the rules to, say, filibuster the motion to proceed. When the chair says Yes, Dems overrule it by a simple majority vote. And so on with the other provisions.

It will be very hard to determine whether these reforms will be effective without seeing their final language. But in the end, the minority will still be able to filibuster on the move to end debate — which is to say the Senate will remain a supermajority institution. Will the reforms discourage filibustering? That we’ll have to find out.

It’s still possible that the simple majority rules change may not come to pass — if Republicans agree to a compromise package of filibuster reforms. But aides say that as of now, no negotiating is taking place.

Either way, what happened today is that Reid, in effect, put his finger on the nuke button. It’s hard to see how he pulls back now.