I am not a Republican (anymore) but am now an Independent. That said sometimes good ideas come from the R side of the aisle and sometimes they come from the D side of the aisle. (Do not get me ranting about the stupidity that has come from both sides of the aisle in the last 16 or so years )
But when you have one party that is considered toxic and evil for that party's leader called your mother and father rapists and drug dealers, or you say if a muslim citizen who was born in the US can't leave the US go to Mecca for a religious sacrament (visiting the holy land) and then return for no muslim should be re-allowed back in the US...
If you have a candidate that states all of these things then it will poison the republican brand for decades. No one would want to ally or listen to them for they already got the bad taste in their mouth. It is an instinctual part of the human brain, the areas dealing with the limbic system are always activated first by visual and auditory cues and these limbic "first impressions" then are used by the more cold rational brain to process the information.
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Donald Trump is toxic for he is like a 1000x bad first impressions, 1000x bad first dates.
Remember there is 19 million US citizens who were not born in this country but immigrated and were legal residents when they immigrated followed by then becoming legal citizens with voting rights.
Donald Trump is toxic for when someone talks shit about your Mom or your Papa and they do not deserve it part of you emotionally reacts to those insults. We have 20 million people who are A) Adults with voting power right now, and they were born in the United States as US Citizens B) But these same adults who were born in the United States have 1 or both of their parents who were not born in the US. Aka their parents were 1st generation immigrants and we would now call their children 2nd generation immigrants even though they lived in the US all of their life and are US citizens through and through.
Even if these people have lived in the US for 18, 20, 30, 50, 70 years, if someone talks shit about your mom or dad who decided to come to the US for a better life... if they talk shit about your love ones and call words that imply they are less than human...then you are not going to like, trust, or stand that politicians. Worse Donald Trump is using inclusive words saying ALL immigrants are like this instead of SOME immigrants are like this. Those small difference in language are actually almost always noticed by most people and it modifies two things 1) How they react to the person, and 2) How they react to what they are saying, so next time when someone says something similar but is not racist it poisons the non racist message due to an unconscious association.
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So in other words we have 40 million voters who are either 1st or 2nd generation immigrants and are legal so you better pick your words very carefully when you are talking about immigration for immigration is not the same thing as illegal immigration. 125 million adults voted for either Obama or Romney in 2012, so even a small change of turnout or one side to the other can make huge differences, for a swing of 5% of 40 million is an additional 2 million vote. 5% extra turnout means 2 million extra votes, A switch from D to R or R to D of 5% is 4 million effective extra votes (since you also remove the same amount from your opponent)
Romney only lost by 5 million votes, so a change of roughly 7% of the 1st and 2nd generation immigrants would have have romney winning the election by a 2 million margin (3.5 million removed from Obama, 3.5 million added to Romney)
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And this is why Donald Trump is going to destroy the republican party. Not the republican establishment, but the entire republican party. He is going to cause a generational shift where some people just can't trust the republicans anymore.
It is like a reverse version of the political science book Whats the Matter with Kansas, a guy who was asking why does Kansas vote republican even though most people would do better under Democratic policies and ideals). In the book the author argues it is culture which defines people's voting and while democratic economic policies may benefit Kansas (and these left wing populist / progressive / agrarian politicans were huge in Kansas prior to 1950s) and that the fact that the Republicans message and style just hits home more due to them talking about cultural issues and the sharing of culture resonates and causes Kansas to vote Republican.
Well its much the same here. If I think you hate my race, religion, sexual orientation, creed, job, hobbies, etc... if you hate things that I associate as core to my being or my origin and how I got to where I am today then I am far less likely to vote for you, and also I am far more likely to support the other guy and encourage turn out against them.
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On another note I normally can't stand political events but there was a thing a few months back in Houston where Hugh Hewitt was going to debate Ann Coulter and the 1994 Pete Wilson Immigration Amendment and the current US presidential candidates including Donald Trump. I had to attend and it was hilarious how much Ann just did not get the immigration issue and how your language over immigrants is not the same thing as immigration policy and you need to cool your jets or you will turn the independents, moderates, non super serious political people (both liberal, and conservative) against your party and thus you will not win elections as evident based off what happen to California since the 1994 California Proposition 187 /Save Our State (SOS) initiative / Pete Wilson's Proposition which meant to target illegal immigration but massively caused an increased voter registration and turnout of any 1st and 2nd generation immigrants which has caused California to be a democratic stronghold since 1996.