above example actually suggests a different point than one you wish to make, although it illuminates that ferguson's approach is more about presidential candidates than it is about political parties...
politcal 'investors' rely upon information and knowledge about candidates in making their investments (giving their financial support), such info includes polling/public opinion numbers of which you are so fond...
re. above, clinton is odds-on favorite to win re-election to her u.s. senate seat, she has ostensibly 'toned down' views that drew health care industry ire, she is considered both knowledgeable and a congressional leader on heath care issues, 'investment' in her senate campaign is rational choice for some/many health care industry honchos...
as 'early donors', health care industry execs expect to get a 'seat at the table' so that they can 'influence her agenda' if she is the 2008 dem prez candidate, this is in accordance with the 'investment' theory of politics...
of course, it's an 'investment' theory and investors do not always get a return, in politics the failure to do so may be the result of others' 'investments', for example, ferguson points out that working people can 'invest' in politics through labor/community/political organizations...
thus, ferguson offers an 'organization' theory as well, his view is that the relative absence/weakness of u.s. working-class organizations has meant that politics is largely played out by rival 'investor blocs' (a theory of which might be called 'plural elitism')... mh