Does he want to guarantee Republicans will stay home and let the Democrats win the election? What part of "Your entire party disagrees with you on this one" doesn't he understand?

The only charitable explanation I can think of is that he's waiting for the recount of the Mexican presidential election to be finalized.

Why should it matter?

Is there a Mexican candidate for whom he would veto the fence?

(Also, I thought that was over by now)

As I said... it's the only explanation of which I can think, it's not necessarily a good one.

Some were claiming that too much perceived anti-Mexican sentiment in the USA would lead to a backlash in Mexico and result in a victory by Lopez-Obrador, the Chavista candidate.

No, the victor has not been officially proclaimed yet.

Guys, the Mexican election is over. The final certification was the middle of last month. The PAN candidate won.