How do you suppose we can challenge their ideology? By feeding them mine? Yours? Just saying "you fuckin wrong!"?
It's not about trying to convince them to believe something else. Challenge them meaning they aren't going to have their rallies and hate speehes untested, that they aren't going to be able to spread their ideology to the youth and disenfranchised, most importantly.
This really has to do with history or preventing history from repeating itself. Facism can only be successful where there is crisis. In all the countries where fascism became victorious, they had, before the growth of fascism and it seizing power, a wave of radicalism of the masses - the workers, the poor and farmers, and even with the bourgeois class. In Italy for example, after the war and before, there was revolutionary wave of tremendous dimensions; the state was paralyzed, the police did not exist, the trade unions could do anything they wanted -- but there was not a party capable of taking power. As a reaction came fascism.
In Germany, much of the same.
Right now in the US, the country is polarized. We have a new wave of radicalism, both with the far right, rebranded as the "Alt-Right", the Left picking up from Bernie Sanders "mainstream socialism" to the masses and shifting it further to the left, a new generation of youth who for the first time in well over 50 years are rejecting capitalism by large number (upwards of 50%).
Capitalism has been degrading for quite some time in the US, the quality decreasing. Already this country has had fascist elements and a new wave of radicalism is emerging. But like the examples above, there does not exist a party for the working class. The private sector is less than 7% unionized, the lowest since the 1920s, meaning the working class is incredibly weak, essentially no power at all. That's a dangerous situation and opens the doors for extremism, like Facism, to grow, gain power and eventually seize power. Trump and the people in his Adminstration are in office, that alone should be a huge wake up call.
Now you must remember, Capitalists will always support Fascists specifically when Capitalism itself is in crisis and on the verge of collapse; it is used as defense against the revolutionary working class from seizing power. Fascism is absolutely great for Capitalists, as Fascism is a movement which emerges in the context of historic weakness for the Capitalist system, in order to safeguard it.
Of course, not all capitalists and the ruling elite would want facists in the driver's seat, but instead use them more as a repressive force against organised working class without having to take state power - in Germany for example it is not because the ruling elite as a whole desired a Nazi government, it is because they believed Hitler could be controlled by the parties of the establishment (smaller example of this is the Republicans and Donald Trump). That Hitler ultimately did seize power is something they didn't expect, but also something they weren't terribly bothered by.
Another factor is Fascist movements use radical rhetoric in order to appeal to the working class. They use the radicalism to their own benefit. An example of this in our own country is Trump saying the Republican Party will be a "workers' party" under his Presidency, a perfect example of how large chunks of even the working class can support the very people looking to exploit them the most.
We are in early stages of crisis and none of us know how long it will last. It could be 25 years from now before real crisis occurs, maybe longer. Or it could be 10 years. No one knows. What we do not want is neo Nazis, white nationalists, the alt-right and straight up fascists growing in numbers (which they are) and becoming a powerful element in this country, for the time will come sooner or later where conditions get bad enough for fascism to seize power.