Earlier quoted context omitted.
Racism is not a political opinion. Political opinions are along the lines of "are my taxes too high" or "do we spend more on parks or on police". A belief in the inferiority of another race isn't political, it's something else. I don't know what, but discriminatory and bigoted beliefs don't belong in political discussions or any discussions about how we govern ourselves or organize society.
Racism does not have to be about "the inferiority of another race". Racism can be "I don't think it's good for the economy that so many people from outside are coming" or "this culture is completely incompatible with ours and I don't want these people here". It's a pretty lax word. By outlawing "racism" you're outlawing discourse.
Saying you don't want immigration isn't racist if it applies to more than just immigration from certain racial groups.
However, her tweet was not about that. She called out Muslims as having certain behaviors which is clearly racist.