While it’s true that there is a new breed of Democrat in the U.S. Senate, and that the senators who have been elected post-Iraq fiasco are infused with quite a lot more backbone than their predecessors, the reason that Harry Reid was finally able to pull the trigger on the nuclear option is because the oldest and longest-serving senators in the body were finally convinced it was necessary. The oldest sitting senator is Diane Feinstein, and she was not an easy sell.

“There are many of us that really wanted to keep things the way they were, because that’s the way they were,” said Sen. Dianne Feinstein (D-Calif.). “One thing I know: that you learn from history. And right now we can’t let the present be the future. So you’ve got to make the change, or this becomes a body that doesn’t mutate.”

That’s kind of an odd way of putting it, especially since mutations are usually a problem and the abuse of the filibuster was basically an unwelcome mutation in Senate norms. Yet, I understand Sen. Feinstein’s point. Sen. Patrick Leahy is the President pro tempore and has served in the Senate for nearly 39 years. He, too, was very reluctant to change the rules, but, as the chairman of the Senate Judiciary Committee, he is responsible for ushering President Obama’s nominees onto the courts. Unless the rules were changed, he could not do his job.

Other long-serving senators went along, too. Among them, Max Baucus (34 years), Tom Harkin and Jay Rockefeller (28 years), and Barbara Mikulski (26 years), all saw no alternative to changing the filibuster rule. Of the true old-timers, only Carl Levin of Michigan could not be convinced that the nuclear option was necessary.

What distinguishes the younger generation of Democrats isn’t so much that they are less tolerant of obstruction, but that they are less likely to let themselves get pushed around. Here’s the youngest senator, talking about dealing with the Republicans’ antics:

“There’s a time to reach across the aisle and there’s a time to hold the line,” said Sen. Christopher S. Murphy (D-Conn.), the body’s youngest member at 40, who was elected in 2012. “And I think so far this year Democrats in the Senate have done a very good job of mixing across-the-aisle compromise with some heretofore unseen spine-stiffening.”

The time has come for Democrats to take a harder stance against the tea party Republicans, he said.

“These folks have come to Washington to destroy government from within and will use any tool at their disposal,” Murphy said. “To the extent that we have the ability to take tools away from the tea party, we should do it. And one of the tools was the filibuster. Another was the belief that Democrats would cave in the face of another shutdown or debt default.”

For Murphy, the failure of the Senate gun control bill earlier this year was the final straw. He took on the issue of gun violence after the Newtown school shooting in his state in 2012. A bipartisan bill crafted by Sens. Joe Manchin III (D-W.Va.) and Patrick J. Toomey (R-Pa.) had 55 votes but failed to advance.

“I was a proponent of filibuster reform coming into the Senate, but I became a revolutionary on this issue when we lost the gun bill,” Murphy said.

The new generation lives in less fear and is willing to fight. Maybe it’s because they weren’t scarred by the Carter/Mondale/Dukakis elections or by the post-Sept. 11 bullying of color-coded terror charts, duct tape, and mushroom clouds, or maybe it’s because the progressive wing of the party got organized online and began to carve out a space in the national dialogue that made it permissible to be a liberal again.

Whatever it was, I welcome this new development.