There’s no doubt that the Tea Party could get the Republicans in trouble in certain Senate races. In Nevada and Kentucky, for example, Sharron Angle and Rand Paul knocked off candidates preferred by the G.O.P. establishment to win their primaries. Although the FiveThirtyEight model has both Ms. Angle and Mr. Paul as slight favorites in the general election, the races are closer than they otherwise might be.
With Senator Lisa Murkowski’s concession in Alaska late Tuesday night to the insurgent candidate Joe Miller, the Tea Party has now played a role in defeating two Republican incumbents (Robert F. Bennett of Utah is the other). In these two cases, the Tea Party is on much firmer tactical ground.
Nobody would mistake Ms. Murkowski and Mr. Bennett for liberal, but they have not been strict party-line voters. Mr. Bennett, like his colleague Orrin G. Hatch of Utah, had not been averse to gestures of bipartisanship. He teamed with Senator Ron Wyden of Oregon, a liberal Democrat, to propose a market-based universal health care bill. And Ms. Murkowski, though not an authentically moderate senator like Olympia J. Snowe and Susan Collins of Maine, was between the fourth and eighth most liberal Republican in the Senate, according to several rankings systems.
Under certain circumstances, these dalliances with centrism might be something Republicans might tolerate. It is unlikely that a senator significantly more conservative than Ms. Collins or Ms. Snowe could be elected in Maine; instead, the seat would probably default to a Democrat. But Ms. Murkowski and Mr. Bennett hail from Alaska and Utah, two of the most conservative states in the country (although Alaska is somewhat idiosyncratically so), and Republicans there could afford to be picky.
It’s possible to examine this a bit more scientifically. The chart below plots the ideological positions of Republican senators. Along the horizontal axis, I have plotted the partisan orientation of the state, ranging from more liberal (left) to more conservative (right), as according to the Cook Partisan Voting Index. On the vertical axis is a statistical representation of the senators’ voting records, according to their DW-NOMINATE scores. These scores run from -1 (very liberal) to +1 (very conservative); the more conservative senators are plotted toward the top of the chart. Finally, the dashed line represents how conservative we would expect a Republican senator to be, based on the partisan composition of her state. The further below the dashed line that the senator appears, the more liberal he or she is, relative to the state. Those far below the line, from a Republican point of view, are arguably not pulling their weight.
Five Republicans stand out as being especially far below the line — that is, they are more liberal than you would typically expect a Republican from their state to be. The list includes George V. Voinovich of Ohio, who is retiring, and Richard G. Lugar of Indiana, along with Mr. Hatch. And, sure enough, we also see Ms. Murkowski and Mr. Bennett.
Should Mr. Miller — along with Mr. Bennett’s successor as the G.O.P. nominee in Utah, Mike Lee — succeed in winning in November, the Republicans will have replaced somewhat conservative senators with very conservative ones without having put much at risk. That outcome appears nearly certain in Mr. Lee’s case, although it is somewhat more tenuous for Mr. Miller, who was only 8 points ahead of the Democratic nominee, Scott McAdams, in an independent poll earlier this week. Still, in an election cycle that is shaping up to be an outstanding one for Republicans, this is a healthy risk to take.
Democrats, meanwhile, mounted serious primary challenges to three of their incumbents: Arlen Specter of Pennsylvania, Blanche Lincoln of Arkansas and Michael Bennet of Colorado (the challenge succeeded only in Mr. Specter’s case). None of these quite fit the paradigm. Mr. Specter had begun to vote as a liberal, but switched to the Democratic Party only last year. Ms. Lincoln, while unabashedly moderate, hails from a red state; the equivalent case on the Republican side would not be challenging Ms. Murkowski, but rather someone like Ms. Collins. (The challenge to Ms. Lincoln had other strategic merits, like her low approval ratings.) And Mr. Bennet had been appointed rather than elected, which made his primary challenge somewhat routine. But if Democrats are looking for examples of which incumbents to challenge in future election cycles, the Tea Party has just provided them with a couple of good ones.
This post has been revised to reflect the following correction:
Correction: September 1, 2010
An earlier version of this post referred to Senator Michael Bennet of Utah. He represents Colorado.