In our Monday Fix newspaper column, we detailed how the Senate -- at least so far in 2013 -- has resembled nothing so much as the House: a deeply partisan body where not all that much gets done.

The U.S. Capitol.

"The Senate has undergone a marked transformation, symbolized by increased partisanship, blockading for the sake of blockading and even some downright personal nastiness," we wrote.

And, we detailed two theories for why the Senate is starting to look like a second House.

1. Almost half (48) of the current Senators served in the U.S. House previously.

2. The amazing inexperience of the current Senate. 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.

After the column published, we heard from lots of folks who insisted we had missed a theory (or two).

1. Conservatives complained that we gave Senate Majority Leader Harry Reid (D-Nev.) a pass despite the fact that the way he has run the upper chamber, they argued, contributed to a heightened partisanship. "He drafts partisan bills, skips the committee process, fills the amendment tree, and then complains when the R's vote en mass to oppose," wrote one unnamed Republican lobbyist. "Of course they do. It's one thing to vote against legislation that violates some policy principle, but it gets easier if both the policy and the principles violate your sense or order."

2. Liberals said we needed to put more blame on former House Speaker Newt Gingrich who introduced the idea of politics as partisan warfare to a whole generation of aspiring office-holders. That may be a bridge too far since we aren't sure Newt invented political partisanship. But, it is without question true that many of the young-ish Republican Senators who previously served in the House, the only style of politics they have known is the era of hyper-partisanship hat Gingrich ushered in during the early 1990s. (One GOPer pinpointed what he believed was the single moment from which it all sprung -- the 1984 election recount in the Indiana House race between Frank McCloskey and Rick McIntyre.)

So, that's four theories. Which one do you ascribe to?