Justices have become increasingly likely to hew a partisan line. I expect that the general trend predates him, but every justice after the independent-minded Souter was closely vetted for ideology.
This is a separate but very interesting question–the speaker is indeed third in line of succession, but what would happen in case of a Cylon attack in which POTUS, VPOTUS, most of the Cabinet, AND the Speaker were killed? The Sec. of Education is next in line of succession, but the rump Congress elects a new speaker–constitutional crisis!
In the case of such a devastating attack the succession might not matter much. Clearly the Founders can’t have envisioned it–they never even intended the whole government to be together at at one time, as for the State of the Union address, which now necessitates one member of the cabinet to go into hiding JUST IN CASE.