For Democrats: Clinton is a centrist. Clinton has her supporters. Sanders is left wing progressive. Sanders has his supporters. Sanders’ supporters will support Clinton in the General. Democratic voters and the Democratic establishment would never support Trump. I assume Democratic voters will get out the vote not only to support Clinton, but to make a statement about Obama’s legacy, and to deny Trump. Democratic voters will also be more energized for a centrist Supreme Court nominee, denied to Obama by the establishment Republicans. The Democratic base and establishment will be unwaveringly supportive of the Democratic candidate.
For Republicans: Trump has his supporters. Establishment and status quo Republicans want anyone but Trump. Cruz has his supporters. Establishment Republicans do not like Cruz. During the primaries Cruz has had only nominal support from status quo Republicans.
If Trump wins nomination middle-of-the-road and establishment Republicans will seek another option, be it an independent candidate, a vote for Clinton, or abstaining from voting altogether.
If Cruz “wins” or is given the nomination at the convention the Republican establishment would be content. However, not every status quo Republican will hold their nose and vote for Cruz. Trump supporters might support Cruz, but it is more likely they would rebel against the Republican establishment that denied their candidate the nomination.
As a side note: Many have noted the cognitive dissonance displayed by the GOP where they assert that the American people must have a say in who should pick the next Supreme Court justice and yet they are willing to undermine the will of their own GOP supporters when it comes to voting for Trump as the GOP nominee.
For Independents: I assume that Independent voters move left or right depending upon their interests and the issues at hand. I assume them to be rational actors seeking the best possible outcome for the nation and, by extension, themselves. I assume Independent voters will, under normal circumstances, be the key voting block to select any president.
I assume most Independents voters dislike Trump. I assume most Independent voters dislike Cruz. I assume most Independent voters are at least ambivalent about Clinton.
The Independent voters also see the chasms developing within the GOP as the establishment, the status quo, Trump and the Tea Party contend for the future of the party. I would expect Independents would prefer to remain outside of this particular fray.
Why a Third Party/Independent Candidate is not feasible: Democratic voters are unified. Independents will act in their self-interest. Republicans will split because their lack of unity would be the reason for a third candidate to enter the race. Any further fracturing or splintering of the right wing/GOP further cements Clinton as the favorite.
As a side note: It is ironic that the GOP had Trump sign a pledge that he would not run as an Independent, and now the establishment is whispering the same thing to somehow stop the Trump juggernaut.
Why Clinton wins:
Trump will attract Trump supporters, most status quo Republicans, and a few establishment Republicans. Ergo, Trump will attract most, but not all Republican voters. Enough will abstain, vote third party if that is an option, or vote for Clinton. Trump will receive some Independent support and no Democratic support.
The GOP establishment seems convinced and is now fearful that it will lose control of their party if Trump is the nominee and they are doing all they can to subvert the will of primary voters.
If Cruz is the nominee he will appeal to even fewer voters than Trump. Cruz will be the compromise candidate no Republican wants and Trump supporters will have a legitimate grievance if the nomination is manipulated to install someone that has garnered so little enthusiasm. Cruz has no conceivable chance of winning
Clinton attracts all Democratic voters, many if not most Independents, and some disaffected middle-of-the-road Republicans. This will lead to a solid victory for the Democratic party, the sustaining of key Obama legacy legislation and a mandate to lead for four to eight years.
Caveat: Despite all of the ruminations above I will, as I always do as the elections get close, defer to the sophisticated assessment of scientific polling (in the manner of Nate Silver’s fivethirtyeight) that will make everything clear once the two nominees have been selected. This scientific approach has proven to be an accurate and objective assessment of the will of the electorate.