In this post, I discussed certain issues regarding the details of how the U.S. could adopt a shadow cabinet system, similar to that in use in the U.K. and other parliamentary democracies.
I doubt that U.S. presidential candidates would be enthusiastic about adopting this idea.
The upside would be that a nominee could ensure the support of a wavering party member, via the offer of a shadow cabinet appointment. Someone like Senator Joe Lieberman, who supported John McCain's candidacy, could perhaps have been brought onside by Barack Obama, if Lieberman had been offered a cabinet post.
In Lieberman's particular case, he was probably too firmly placed in the McCain camp by the time shadow cabinet appointments might have been made, in August. But if there were a similar situation in the future, a presidential nominee could try that.
The flip side of that coin, however, would be that potential cabinet choices might lose their enthusiasm for supporting the fall campaign, if they had been passed over for a shadow cabinet appointment in August. If Obama had publicly announced Hillary Clinton as shadow secretary of state, Bill Richardson may have been miffed. Richardson had been mentioned as a candidate for the State Department, and perhaps it was just as well from Obama's standpoint that it was after the election when he gave Richardson what many consider to be his consolation prize: the Commerce Department.
On balance, I think the number of disappointed would-be cabinet members will always be larger than the number of shadow cabinet appointees. So, presidential nominees will probably not deem it to be in their interest to make cabinet appointments before the election.