The problem you have with your entire premise is the fact that the behavior of the Democrats is the exact opposite of what your bogus polls cited say they should be doing.
Actions speak louder then BS Polls. They fear Trump.
You don't like the polls, so they're bogus. You're just seeing what you want to see. The polls seem to reflect what I personally hear from other voters I know, so I don't really have any reason not to believe them, especially since they've generally been consistent regardless of who is doing them.
But beyond that, I assume your logic is that if Trump is really that bad a candidate, the Democrats would want him to win the nomination, and therefore wouldn't be criticizing him so much. There are two logical problems with that.
The first is that people aren't always free to act as strategically as you think they should. At this point, the Democrats are still locked in a primary fight. Trump's statements have become so prominent that the Democrats are forced to comment on them. And to satisfy their own base, and to appeal to their own voters, they have to come out swinging against him. Attacking Trump gains them goodwill with their own primary voters. Hillary, in particular, is attempting to establish herself as the presumptive nominee, and part of the way she conveys that is by attacking the GOP frontrunner.
The other problem is that they may well have concluded that Trump is going to be the nominee, so the "let's hope they nominate the worst candidate" time has passed. They're going after Trump now because they want to set such a strong negative image in the minds of the voters that Trump has no hope of repairing it.
Either way, them attacking Trump does not prove he's the most electable Republican. That's a silly conclusion born out of pure pro-Trump bias.