legislate against (something)
To ban, prohibit, or forbid something through legislation. As it stands, over 20 states still legislate against recreational marijuana, despite the federal government lifting its own ban earlier this year. I just don't think the government should legislate against things like cheap junk food.
legislate for (something)
To allow something to happen or take effect through legislation. The president has been a staunch supporter of healthcare reforms since taking office. Congress has successfully legislated for raising the age at which one is allowed to purchase alcohol.
legislate against something
to prohibit something; to pass a law against something. You can't just legislate against something. You have to explain to people why they shouldn't do it. The Congress has just legislated against insolvent banks.
legislate for something
to pass a law that tries to make something happen. The candidate pledged to legislate for tax relief. We support your efforts to legislate for lower taxes.