"Everything is up for grabs"

http://idioms.thefreedictionary.com/up+for+grabs

referring to the link above "up for grabs "is an idiom means:

available for anyone; not yet claimed

The election is up for grabs. Everything is still very chancy. I don't know who will get the promotion. It's up for grabs.

in total chaos

This is a madhouse. The whole place is up for grabs. When the market crashed, the whole office was up for grabs.

The first definition doesn't really fit the quote, because when Trump has claimed the Presidency it's not available anymore!

Also note that the idiom "up for grabs" exists in both American and British English with the same meaning. Reference: http://www.macmillandictionary.com/dictionary/british/up-for-grabs http://www.macmillandictionary.com/dictionary/american/up-for-grabs

Hope it helps you

Besides available, "up for grabs" also means "not firmly decided"; presumably the etymology for this was something like "available to be changed".

I can't find a dictionary definition saying this, but consider these sentences taken from Google books.

1989

Even if this is so, it is still up for grabs whether God is in some sense posterior to some of His constituents

1989

Between this beginning and end Pip's destiny seems up for grabs as he is lured away from virtue by the ignis fatuus of gentility

1999

Nothing seems to be sacred. Nothing is forbidden. Everything is up for grabs.

If you substitute available for up for grabs, these sentences don't make any sense.

So your sentence means:

I don’t want to think about that possibility, but if it should be, then everything could change.