Brian Moynihan is facing a make-or-break moment tomorrow in his quest to become head of America’s largest bank, as he and two other Boston-connected executives head to Washington for a congressional hearing on Bank of America’s controversial takeover of Merrill Lynch.

Moynihan, a former top executive at FleetBoston Financial and now head of Bank of America’s powerful retail unit, has been considered a top contender to replace Ken Lewis, the embattled chief executive of the Charlotte-based banking behemoth.