"Lifer" defined as someone who dies in office, dies immediately upon leaving office, or retires at a very old age?
Lifer is someone who intends to die in office. Typically they don't, as they eventually get pushed out during their old age as both the party's identity and the identity of the area they represent start to shift beneath them. Examples of lifers who did hold on their whole lives include Robert Byrd and Dan Inouye. Examples of lifers whose districts shifted away from them might include Nick Rahall and Ike Skelton, elected to heavily Democratic districts that became more Republican as the years turned into decades. Examples of lifers who got scared by possible defeats include Jay Rockefeller and Chris Dodd, who overstayed their welcome. Examples of lifers who got old, rusty, and unable to compete include Alan Mollohan and Ralph Hall, who were defeated by ideologically similar but much younger opponents in primaries.
Note that I put a West Virginian into every category -- historically, the "lifer" attitude has been a big part of the state's political culture, though with the recent shifts we'll have to see if that'll continue to hold true.
