The Senate has been looking more and more like the House in recent years, and Thursday's vote to change the rules to allow judicial and executive branch nominees to be approved by a simple majority (a.k.a. the nuclear option) may well be the final step in that transformation.

So, how did the "world's greatest deliberative body" turn itself into something closely resembling the majority-rules raucousness and party-line posturing of the House? The change didn't happen fast. And no single one of the events that led to it would have, in a vacuum, brought us to this place. But, when all of the steps are considered together, it's actually relatively easy to see how we ended up here.

Step #1: Massive changes in the makeup of Congress

Since 2008, 40 new Senators --20 Republicans and 20 Democrats -- have been elected. Six years ago, 44 senators had served at least three terms; today that number is 32. At the start of the 113th Congress, more than half of the senators had served one full term or less. There are 55 members of the Senate who have never lived in anything other than a Democratic majority and a Republican minority. And, many of the newly-electeds come either directly or indirectly from the House -- 48 of the 100 Senators have served in the House at some point -- and have brought a more contentious approach with them.

"Large numbers of senators are former House members and try to turn the Senate into a tightly structured second House,” former Louisiana senator John Breaux told me in April. “They get over that after a couple of years, as I did, but the turmoil it creates can cause dysfunction."

The long-time lions of the Senate are also no more. Since the 2008 election, here is just a sampling of the senators who have gone: Ted Kennedy (D-Mass.), Ted Stevens (R-Alaska), Daniel Inouye (D-Hawaii), John Warner (R-Va.), Robert Byrd (D-W.Va.), Pete Domenici (R-N.M.), Arlen Specter (R-then-D-Pa.), Olympia Snowe (R-Maine), and, of course, Vice President Biden (D-Del.). That's a massive amount of institutional wisdom -- and tradition defense -- to walk out the door in the last five years.

Add it all up and here's what you have: A relatively inexperienced group -- on both sides of the aisle. That level of newness to the Senate also means that there is a decided lack of across-the-party relationships that were once the hallmark of the Senate. Even within parties, relationships have broken down -- witness the repeated back and forth between Arizona Sen. John McCain and Texas Sen. Ted Cruz.

Step #2: The rise of the the tea party -- and its corresponding outside conservative organizations

For a very long time, it was virtually impossible to beat a sitting Senate incumbent unless that person had essentially disqualified himself by his actions. (See Bob Smith's short-lived presidential run as an independent in 2000.)

The rise of the tea party in 2009 and the subsequent 2010 midterm elections changed all that. In 2010, Utah Sen. Bob Bennett and Alaska Sen. Lisa Murkowski both lost to little known primary challengers who were heavily backed by tea party groups and other outside conservative organizations. (Murkowski went on to retain her seat in the Senate by winning the general election as a write-in candidate.) Two years later, Indiana Sen. Dick Lugar, who had served in the chamber since 1976, lost a primary challenge to Richard Mourdock. Mourdock's entire candidacy was premised on the idea that Lugar had "gone Washington" -- from his voting record, which Mourdock said was far too moderate, to his residency.

Those defeats cast a chill over Republicans in the Senate. John McCain, a noted pragmatist, turned into a conservative ideologue in order to fend off a primary challenge from his right in 2010. Ditto Orrin Hatch in 2012. And now, a number of GOP senators -- from Mitch McConnell to Lindsey Graham to Thad Cochran -- are facing tea party-backed primary challenges in 2014. The incentive to work with Democrats on, well, anything has largely been eliminated. And, in fact, there is now a disincentive to cooperation.

Step #3: The relationship of Harry Reid and Mitch McConnell effectively dissolved

The thing that had kept Democrats from invoking the nuclear option prior to Thursday -- they have, after all, controlled the Senate since the 2006 election -- was Reid. Talk to his top aides -- current and past -- and they would emphasize that he was, at heart, a traditionalist who understood that changing the rules was a major move with long-lasting implications for the chamber.

But, that began to change as his relationship with McConnell soured. As WaPo's Paul Kane wrote:

The rift traces to 2010, when Reid thought McConnell wasn’t upfront about how aggressively he would try to help defeat the Democrat in his tough reelection race in Nevada. McConnell, now dealing with a difficult campaign of his own in Kentucky, is incensed that Reid appears to be more than returning the favor. In recent years, McConnell has gone around Reid to cut deals with Vice President Biden.

By this summer, it was clear that Reid and McConnell were at daggers drawn. When Reid pushed the nuclear option in July, he and McConnell got into an extended rhetorical battle on the Senate floor that led McConnell to predict that if Reid invoked the nuclear option he would "be remembered as the worst leader here ever." Reid eventually relented, bowing only when Republicans -- led by McCain, not McConnell -- made concessions to confirm a handful of appointments to things like the National Labor Relations Board.

With no personal relationship to draw on to find a deal, the use of the nuclear option became a matter of when, not if.

Step #4: Thursday's nuclear option vote

Yes, the rules change passed Thursday applies only to executive and sub-Supreme Court judicial nominations made by the president. But, if you think that such a narrow reading of the rule change will last long, you haven't been paying much attention to politics in the last decade or so.

This is a Pandora's Box moment for the Senate, a vote that will be forever referred to when future majorities adjust the rules on a particular subject to allow the majority to rule. The minority in the Senate always had far more power than the minority in the House due to the 60-vote cloture rule. No more.

And, aside from the practical implications of the rule change, which seem easy to predict but will continue to play out over the next handful of Congresses, there is the symbolic poisoning-of-the-well effect it produces. While the argument could be made that the relationship between the two parties can't possibly get any worse, there is now even more -- if possible -- ill will between the two sides. Moments after the vote, McConnell and Tennessee Sen. Lamar Alexander promised retribution for this move at the ballot box next November. Even Republicans like Alexander, McCain, Bob Corker and Susan Collins, who had shown a willingness to do some work with Democrats, will be scalded by this vote. In short: If you thought the last few years were bad, you ain't seen nothing yet.