If you win an election by a 3:1 margin, are you the predominant winner or the predominate winner?

Predominate is a verb meaning to prevail, to have greater importance or quantity. You can easily see the root word: dominate, to command over. Its history is uncertain, but it was probably borrowed from Medieval Latin's predominare.

Queens and Staten Island, where single-family homes predominate, actually saw their first upticks in years.

Tomorrow and the day after tomorrow, winds from westerly directions predominate.

They gave an insight into the ethos that predominates at top English clubs.

Predominant is a derivative of predominate and, by extension, dominate. Predominant, however, is an adjective describing something or someone with the greatest importance, influence, or power. It can also be used to describe something that is most common.

The fancy shovels that didn't really cost that much were an example of the predominant theme of Monday's ceremony.

Just as in years past, steel was the predominant material at this year's NAHBS.

Such direct calls for regime change may not represent the predominant public opinion in Syria.

The trick is not to use predominate as an adjective, as some less-observant language users are wont to do. It's often labeled as a needless variant by usage manuals and dictionaries. Garner's Modern American Usage notes that the usage of predominate as an adjective is at stage 3 of language change: "the form is commonplace even among many well-educated people but is still avoided in careful usage." Your best bet is to reserve predominate as a verb and predominant as an adjective.