seal the nomination

[xinyi1356]

The following is taken from Super scary Tuesday: Trump v Clinton
A dozen states hold primaries today, in the candidates’ biggest test so far. None will seal the nomination by tonight. But the number of delegates awarded will (if polls are right) give Donald Trump and Hillary Clinton such overwhelming leads that the eventual result is a foregone conclusion.

Can you tell me the meaning of the phrase in red? Thank you!

 

[grassy]

There won't be announced any official candidates yet.

 

[BLUEGLAZE]

Imagine you have finished writing a letter and you are now going to send it to the intended person. You must put it in an envelope and seal it.
No one else, by law, can open the envelope except you or the recipient. Sealing the nomination means no one else can be number one. You are the chosen one. Like an envelope being sealed (closed). Voting is now closed (sealed).

 

[Englishmypassion]

Blueglaze has explained the literal meaning well.
In other words, none will secure/win the nomination (for themselves) by tonight.