When you're sure to become your party's candidate for office, do you clench, clinch or cinch the nomination?

Clinch, I believe.

“Clinch” is the most common, and probably what they were saying on NPR. Interestingly, though, all three are appropriate.

Clinch. NPR is not my hero.

Clinch.

Clench your jaw. Clinch the nomination, It’s a cinch the Steelers will let me down again next season.

NPR kept saying Donald Trump had grabbed_ the Republican nomination by the balls today…squeezed with all his might and ain’t letting go any time soon,

Cinch (slang) To get a sure hold upon. -from Merriam-Webster’s New Collegiate Dictionary, 1961 edition.
I did know, just looked it up to get the exact definition. It’s the term I always heard used.

What one really clenches is one’s sphincter, after a night of Guinness and three bean chili.

Clinch or cinch would work equally well.

win

I’m sure NPR was saying “Clinch”, as most of you knew or guessed. I’m not even 100% sure I knew ‘clinch’ was a word.

I believed they were saying “clench” which might be acceptable (but odd to me), and meaning “cinch”, which fits right in with @kritiper‘s Merriam-Webster’s New Collegiate Dictionary, 1961 edition. Generational thing, perhaps.

Thanks, all. The question turned out exactly as I’d hoped.

You clench your fist before you punch someone , but you clinch a nomination or a win.

Now it’s about this time he will create his dedicated “Trumps Troops ’ He will say they are for his and his supporters security but they will look intimidating in their matching shirts and arm bands.