Political parties are dead; they've been dying for many years. Since Kennedy, presidential candidates have run almost independent of parties, which has contributed to this trend.
Parties were useful in organizing governing coalitions in a new nation; we now have other means of doing so, primarily the media and, now, the internet.
Another important contributing factor has been the inability of parties to achieve anything. Consequently, most now believe not just that political parties can't do anything, but that they prevent doing anything.
Presidential candidates increasingly will need to campaign across traditional party lines. This will include informal cooperation ("endorsement"?) of office holders in other power centers (e.g., presidential and congressional candidates) without reference to party. Essentially, it represents building an issue-based mandate that can be used to accomplish objectives in the brief window immediately after an election.