In nature, a tsunami (Japanese term) is a giant wave. Properly speaking, "a tidal wave," used as the equivalent of tsunami, is incorrect; a wave created by a tide can be very, very tiny.
In politics, as well as other settings, "a tsunami" or "a tidal wave" (such as a tidal wave of support) means a powerful trend that, temporarily at least, changes the proverbial landscape.
Example: "(Party X), deeply unpopular in the polls, faces a potential tsunami at the next election, sweeping it out of power."
The phrasing may vary, but this is the general idea.