I never liked open primaries.

[question everything]

I've always thought that we, party members - of all parties - should determine the candidates. Have worried about opposing party disrupting primaries.

And now, when Trump will be the only major candidate on the Republican primaries - assuming that all states would even bother to hold one - I can see many Republicans casting their vote in Democratic primaries to disrupt.

How can we prevent this?

[pnwmom]

1. Good question. I'll wait with you to see if anyone has an answer. n/t

[dogman]

2. The parties would have to hold their own primary, caucus, coronation or whatever.

If they rely on public funding they have to follow the states election rules.

[MichMan]

3. We can't

Crossing over is always a possibility with an incumbent. In Michigan, you just have to state which parties ballot you want when you enter the polling place, so easy to do if someone wanted to try and pick a weak candidate for the opposing party.

[Sherman A1]

10. Precisely

and not really sure that we should try to prevent it.

[manor321]

4. Primaries should be closed

Not much we can do at this point since it would be unlikely to change state's laws at this late stage.

Here is a list of DEM primaries and if they are open or closed:
https://www.thegreenpapers.com/P16/D-DSVE.phtml

[TheBlackAdder]

5. Same here. If folks want to select a Primary candidate, they should declare themselves.

.

It also helps to make them put skin in the game, and reinforce their party affiliation.

.

[loyalsister]

6. It's pretty early to assume Trump won't have a primary

In MO there are always challengers. Most of them symbolic but still make it on the ballots. Republicans will have a choice to resoundingly endorse Trump or they could at least register some dissent. It seems unlikely that they would throw it away in an effort to influence a Democratic primary that will be diluted by 49 other state vote contributions.
It would take a massive coordinated effort to get millions of people in varying states to abandon their opportunity to at least register an opinion on an extremely controversial candidate.

[question everything]

7. The only hope is to have other competitive Republican races

thus voters will have to request the Republican ballot for senator, or member of congress, or state offices..

[loyalsister]

8. Then the question is?

Are there really enough states with enough people willing to participate in this conspiracy for it to have an effect?
I think not.

[JustAnotherGen]

9. There's nothing I can do

I live in NJ and its always decided long before June.

[question everything]

11. Not always. I think that both Hillary in 2008

and Sanders in 2016 conceded after June. After the CA primaires.

[JustAnotherGen]

13. It's going to be too late next year

The CA primary is much earlier in 2020

[TSIAS]

12. Eliminate primaries all together

This election is too important for voters to mess it up. Allow state elected leaders and elders to select the candidate.