The point of this article is that the Republicans are testing various attacks against Hillary because SEE BELOW QUOTE she was in July a prohibitive favorite. It is not related to any decision that she was the BEST candidate against republicans (the elect ability argument she is making and which is the question of this OP).
If Bernie beats her, the republicans will spend the same millions attacking him because he will be the nominee. Everyone knows that Hillary has been attacked for years although you will notice that she was NOT attacked when she was not running for president. However, when she started running for the 2016 presidency (even though rejected in 2008) she was assumed by every pundit and person in this country to be a prohibitive favorite. There was no reason to attack O'Malley or Bernie. They were considered afterthoughts kind of like the fact that the democratic strategists have not been spending money to develop strategies against Rand Paul because he was thought to have no chance.
Now the quote.
"Democratic political groups, of course, will undertake their own offensive to attack a Republican nominee or front-runner, much as they did with Mr. Romney. But their task has been made more difficult by the large and uncertain Republican field.
Both sides agree that the work undertaken long before the election, often in the year before it, creates the foundation for the most damaging attacks."
With respect to your assumption that it is only republican attacks that have shaped her unfavorables in the democratic left. The article does not say that. It says they are trying to attack her on what is her weaknesses. They did not create her image as dishonest. Hillary did. Of course the republicans attack on her honesty. It does not mean that Democrats who see this dishonesty are being misled. AND THE ARTICLE DOES NOT SAY WHAT YOU ARE SAYING.
"But many, essentially, struck the same theme, depicting Mrs. Clinton as untrustworthy, an image that even Democrats supporting the Clinton campaign acknowledge is a weakness. About 57 percent of Americans do not believe Mrs. Clinton is honest and trustworthy, according to a CNN poll released June 2.
“She’s got an open wound, and part of our job is to pour salt in it,” said Glen Bolger, a co-founder of Public Opinion Strategies, the Republican polling firm that conducted the focus groups.
Mrs. Clinton’s allies point to relatively low trust numbers for Mr. Clinton in the 1992 and 1996 elections and his ability to win voters despite his personal failings."
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