The correct answer is: "taxation".
The goverment has the power of implementing changes in the taxation system, in order to raise funds that they can direct to any desired objective. Taxes are the principal means for collecting public income.
The attitudes of the members of either the Republican or Democrat party are not in the hands of the goverment (independently of the party membership of its president), and also the sucess of a trade agreeement needs the conformity of the US goverment and a second party, whose final resolution is not in hands of the first party mentioned.