When and how will a candidate break 55% again?

[Mister Mets]

This probably needs a popular centrist running in a very favorable environment. A subpar opponent would help.

McCain could have done if Bush's approvals ratings were 50% rather than low 30s.

Democrats have picked candidates who have ceilings. Kerry was an antiwar protestor from a very liberal state. Obama was a liberal activist from an increasingly liberal state, although you could imagine him reaching 55% against a George Allen type.