Here's an extremely thorough article by Alexander Bolton of The Hill, laying out, among other things, what Arlen Specter's defection will mean for senior Democrats and their committee assignments.

The key takeaways are as follows:

• Reid and Specter struck an agreement of sorts that Specter will enjoy the same level of seniority at the start of the 112th Congress that he would have if he'd been a Democrat since 1980 when he began his career in the Senate.
• This makes Senior Democrats (who could find themselves bumped from potential committee and subcommittee chairmanships, etc.) very unahppy.
• And in a way, that bodes poorly for Specter, since decisions like these belong to the caucus, not to the majority leader.
• But in the end, the caucus will likely base its decision, to a significant degree, on how he behaves between now and January 2011. And that's yet another source of pressure on Specter to get with the Democratic program.
• In the meantime, the Senate needs to figure out what to do with him right now. He needs committee seats, even if non-leadership seats, and that either means bumping junior Democrats; or forging an agreement with McConnell to adjust the committee ratios based on the new, larger Democratic majority; or adding one Democrat and one Republican to each of the committees Specter serves on to create room for him.

That's a lot of moving parts. And, as you'll see if you read Bolton's article, Democrats aren't being particularly shy about expressing their concerns. Much of this will be decided in the coming days, and we'll bring you any important updates.