I can't speak for him, But my sense is that there will be bigger resistance and harder to implement from the top as it expects people who have already been voting to go through some hoops and will become contentious as voter suppression. Doesn't sound practical. But for 18 year olds, they are just getting started and change has to start from there. Having seen the contents of these tests online, I am certain any high school kid will be able to clear it with minimal effort as long as they are vested in it. This is my understanding.
I'm quite sure there will be unprecedented resistance from all over the world over implementing it for 18-25 year olds as well, since it's a very clear infringement on voting rights, but with his rationale (wanting to make sure only informed people vote) a much better case could be made for people from states with poor educational records.
And even then, politics and (inter)national policy making is very complicated. This civics test that any high school kid can clear doesnt even begin to scratch the surface of measuring someone's understanding of politics.
Encouring young people to take an interest in politics is great, but putting obstacles in front of people before they can vote is a ridiculous notion, especially in light of the spirit of the US' founding fathers. That's what makes it so ridiculous, Vivek is talking about heritage and people needing to understand the principles on which the country was build, but this idea is the exact opposite of that.
And once again, one of the principles of the colonists; no tax without representation. Are people who fail the test no longer obliged to pay taxes? Because a government levying taxes from people who aren't allowed a say in politics is as unamerican as French fries with mayonaise.
Of course it doesnt matter, because even if he is elected (he has no chance), there is no way he will ever (ever ever ever) get this through. Of course he knows this, as all populists do.