When will the candidates start vetting potential running mates?

[Frodo]

Interesting question, especially because this election cycle has been so different.

On the Democratic side, I imagine that Clinton already has a vetting process going on quietly (and unless she is facing Kasich, my bet is her choice is someone like Sen. Kaine).

If Hillary is to have any hope of reuniting the Democratic Party and reconciling Sanders' supporters to her nomination, she will more likely pick a progressive (not a moderate) as her running-mate.  

By this point, that should be a given.  

[Frodo]

Interesting question, especially because this election cycle has been so different.

On the Democratic side, I imagine that Clinton already has a vetting process going on quietly (and unless she is facing Kasich, my bet is her choice is someone like Sen. Kaine).

If Hillary is to have any hope of reuniting the Democratic Party and reconciling Sanders' supporters to her nomination, she will more likely pick a progressive (not a moderate) as her running-mate.  

By this point, that should be a given.  

That really depends on circumstances. Note that I said "unless she is facing Kasich (i.e. a more moderate opponent)". If she is running against Trump or Cruz, she will NOT need a "progressive" VP pick to energize the base. Could you really see any Bernie supporters vote GOP or even abstain in a Clinton-Cruz or Clinton-Trump matchup for example?

There is no better way to lose an election than to give your supporters (especially those who supported someone else in the primaries) the impression that you are taking them for granted.  I can easily see some of Sanders' supporters abstaining or voting third party.  Why not minimize the possible defections by picking a running-mate whom they regard as being one of them? 

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Hillary already represents the moderate and conservative (i.e. older) elements of the Democratic Party.  She needs a running-mate who represents those she hasn't been able to win over during the primaries. 

[Frodo]

Out of the people who have actually endorsed Sanders, Grijalva, Welch, and Peterson are nobodies, Ellison will lose votes because of his religion, Gabbard is far too similar to Palin, and Grayson is dangerously insane. Kaptur could help out in Ohio, but that's the only place that I know of. She should instead go with a progressive who's stayed neutral, or endorsed her.

Like Hilda Solis?  

[Frodo]

Out of the people who have actually endorsed Sanders, Grijalva, Welch, and Peterson are nobodies, Ellison will lose votes because of his religion, Gabbard is far too similar to Palin, and Grayson is dangerously insane. Kaptur could help out in Ohio, but that's the only place that I know of. She should instead go with a progressive who's stayed neutral, or endorsed her.

Like Hilda Solis?  

Solis doesn't have the name recognition. Elizabeth Warren could work.

Hilda Solis is admittedly a dark-horse candidate, but once people get to know her better, she will work wonders for Hillary.