Can I ask my adviser for a raise? Due to the recently passed tax bill by the United States Senate, graduate students will incur more taxes on their income which is already small. This will certainly affect my academic performance and research productivity. One of the professors in the department promised his students to compensate them by supplementary scholarships or awards. Can I ask my adviser to do the same thing to me?
First, it is not quite a law yet as it still needs to go through conference and be signed by the president. Regardless, you can ask for a raise, but arguing for a raise on the basis that your personal situation has changed is not really a good strategy. Most people ask for a raise when they can make a case that they have been performing well.

Many advisors do not have spare money floating around. Stipends can be set at the department or university level. As I said in this answer graduate student tution is an odd thing. Universities may either automatically increase stipends or decrease tution. It is really too soon to know. Your best bet is to probably unionize and make sure the university begins treating students responsibly.

The issue, at least as I see it, is that advisors need to justify to funders why a PhD student is needed as opposed to a permanent RA or post doc. When PhD students are cheaper than the other alternatives, it is easy. In my field, the NIH post doc rate is about $40,000 a year. This means there is only a little gap in costs between a PhD student, after covering tuition and fees, and a post doc.

The problem is not really the change in tax code, but rather in the fact that full time researchers are being charged tuition. This is something, at least in my opinion, that needs to be addressed on a university/department level and not with the individual advisor.