Scott Brown won the special Senate election in Massachusetts over Martha Coakley yesterday. In an earlier post, I argued that Scott Brown’s legislative record in the State Senate put him in the liberal wing of his party – a state legislative caucus which was amongst the most liberal Republicans in the country.

My comparative research design allowed me to place him to the left of Dede Scozzafava, even though the two had never served together. It also allows me to put Brown on the same ideological scale with members of Congress, so I can make an informed prediction about how he’ll vote when he joins the Senate.

In particular, based upon his voting record in the Massachusetts State Senate as well the Votesmart surveys of MA state legislators (include his own from 2002), I estimate that Brown is to the left of the previously leftmost Republican in the Senate, Olympia Snowe of Maine (see her issue positions here) and to the right of the rightmost Democrat in the Senate, Ben Nelson of Nebraska (issue positions here). Just as important, Brown stands to become the pivotal member of the Senate—that is, the 60th by rank most liberal (equivalently, the 41st most conservative)–a distinction previously held by Nelson.

These figures come thanks to the hard work of UCLA political scientist Jeff Lewis who keeps an archive of almost up-to-the-minute votes in Congress, and Stanford political scientist Simon Jackman who calculates current ideological scores for all members of Congress in the current 111th Congress, past and present.

By dropping Senators no longer in office in this Congress, I created a dual Google Docs spreadsheet ranking of current US Senators before Brown’s election, and afterwards. It can be found here. The crucial columns to focus on are the first one, containing the ranking, and the third one, containing Jackman’s best estimate of their ideology (the final three columns express the uncertainty surrounding the estimates – ignore this for now). For a graphic version of this data, click here.

Before yesterday’s election, the 60th senator was Ben Nelson. This ranking made Nelson uniquely powerful–in political science we call this power pivotality. It’s a very useful insight into how Congress works. Nelson was pivotal because, on any divisive legislative votes, his vote could either provide the crucial 60th vote to cut off a filibuster, or the crucial 41st vote to sustain one. And of course, without the Senate, there’s no new law. Thus, it’s not a surprise that Nelson was such a central figure in the health care debate, and why the President expended no little effort in trying to convince Nelson to come on board, which he finally did in the wake of a sweetheart deal for his state.

After Brown’s election, however, the picture changes. Paul Kirk–the appointed temporary replacement for Edward Kennedy–estimated to be the third most liberal Senator, leaves. Brown, who’s to the left of Snowe but to the right of Nelson, enters. He therefore becomes that pivotal 41st vote to sustain a filibuster and deadlock legislation (or the 60th vote to end a filibuster and pass it).

How far to the left of Snowe and how far to the right of Nelson is Brown? It’s difficult to tell exactly. In the spreadsheet, I put his score (in Jackman’s scale) at 0.299, or a smidgen to the left of Snowe (0.300). But he could just as well be just a touch to the right of Nelson (0.138), too. And his drifts left and right will be watched very carefully by President Obama and Congressional and party leaders, given his likely newfound status as the filibuster pivot. That’s a lot of power.

Thus, I disagree with Josh Tucker that the election isn’t that consequential. First, the pivotal Senator will now be a Republican, not a Democrat. The parties put a lot of pressure on moderate members of Congress to vote one way or the other; it’s often unsuccessful, but its a pretty powerful source of influence. Second, that pivotal Senator will be Brown, not Snowe (if my prediction proves accurate). Finally, this pivotality will exist on every issue, not just health care reform, which probably just expired in its current form. Not too shabby as a consequential election, right?

Could I be wrong? Yes. It’s possible Brown turns out to be more conservative than we would have expected given his rather liberal state legislative record. At last night’s victory speech, he laid out a number of conservative policy positions.

The observation that politicians are ideologically consistent as they move throughout their careers (“they die with their ideological boots on” in Keith Poole’s memorable phrase) is true on average, not in every case. What could make him more conservative? The party could pull him in that direction. Or maybe presidential considerations (Obama has made the hearts of all State Senators full with ambition) will pull him to the right.

But let’s be realistic. Scott Brown is a politician, not a kamikaze pilot. As David Mayhew argued in 1974, the first and proximate goal of politicians in the United States is to get re-elected. Brown will have a far harder time in 2012 against some credible, seasoned Democrat who won’t get surprised again (or run so badly). Turnout will be higher in that presidential year, meaning the Democratic base will be far more evident at the polls. And the Democrat will get to ride Obama’s coattails, influencing independents in the Democratic direction. And Brown doesn’t have that many years to build up the incumbency advantages that other freshman Senators get. He won’t have brought home as much bacon, and he won’t have risen too far in Congress.

All in all, 2012 will be a very tough election for Brown. So what will the soon-to-be-worried Senator do to enhance his electoral chances? He’ll take the public opinion pulse of his state very, very carefully. And his state is amongst the most liberal in the country. Unless he aims to run for President in 2012 (pro-choice Republicans do well in Republican primaries, right?), his liberal constituency and a desire for re-election will inevitably pull him to the left. Sure, he is far more conservative than Kennedy, Kirk, or Coakley, but that’s immaterial. Brown’s a liberal Republican, and now he’s pivotal.