The basic story is simple. Roosevelt was looking for justices who would uphold the New Deal's major initiatives or, more generally, who held expansive views of the scope of the national government's powers under Article I. He was relatively indifferent to his nominees' views on issues of civil rights and civil liberties, as his nominations of Reed and, later, James Byrnes show. (I plan another post about the Hughes Court's civil rights and civil liberties cases prior to 1937.) But, I think purely as a contingent matter -- that is, not intrinsically related to their views on national power -- some of Roosevelt's nominees happened to hold liberal views on those issues. Yet, some of the nominees, especially Frankfurter and, again later, Robert Jackson, defended their positions on the scope of national power not with an affirmative case for national power but with a negative case about the appropriate role of the Supreme Court in a democratic government. That negative case, they believed, had implications for the Court's role in civil liberties and civil rights cases.
In the years after 1937 the Court, now "controlled" by FDR's appointees, struggled to develop a coherent account of the Constitution and the Court's role in enforcing it that would simultaneously affirm broad national power and give the Justices some role to play in enforcing civil rights and civil liberties. Today we tend to think, or so I believe, that Harlan Fiske Stone solved the problem early on with Carolene Products' Footnote 4, but within the Court Footnote 4 was only one of the options on the table. Basically, or so I am likely to argue when my work is done, the Roosevelt Court floundered in its early years: The Justices knew pretty much where they wanted to come out, but had an extremely difficult time coming up with an account of their role that satisfied them when they reached the conclusions they did.
I mentioned Elena Kagan in this post because my sense -- based on no inside information, of course -- is that her selection reflects a strategic calculation by President Obama similar to FDR's. What the President wants is a Supreme Court that will stand aside when or if Congress enacts the programs the President favors, and is relatively indifferent to his nominees' views on other questions. Put pretty crudely, SG Kagan hasn't been vetted for her views on what we might call "Warren Court" (or "Brennan-Marshall") issues (other than the scope of national power) -- which may be why some on the liberal-left side of the spectrum are nervous about what she would do as a Justice dealing with those issues. (A minor point that I haven't seen made elsewhere: Even assuming all the somewhat critical things some have said about the way she structured her career, her first career choice was to decide who to clerk for on the Court of Appeals. Again, assuming that she wanted to get a Supreme Court clerkship, when she made her choice there were a number of "feeder" judges, some quite liberal, some less liberal, some conservative, and some extremely conservative. She chose to clerk for Abner Mikva, about as liberal a feeder judge one could have found when she made her choice. I for one (to use a locution favored by the Justice she clerked for) think that provides some indication of her views on Brennan-Marshall issues.)