In his “Opinion on the Constitutionality of a National Bank (1791) Jefferson argued that the formation of such an entity would allow Congress to "take possession of a boundless field of power” which would give them the means “to do whatever evil they please”:
To lay taxes to provide for the general welfare of the United States, that is to say, “to lay taxes for the purpose of providing for the general welfare.” For the laying of taxes is the power, and the general welfare the purpose for which the power is to be exercised. They are not to lay taxes ad libitum for any purpose they please; but only to pay the debts or provide for the welfare of the Union. In like manner, they are not to do anything they please to provide for the general welfare, but only to lay taxes for that purpose. To consider the latter phrase, not as describing the purpose of the first, but as giving a distinct and independent power to do any act they please, which might be for the good of the Union, would render all the preceding and subsequent enumerations of power completely useless.
It would reduce the whole instrument to a single phrase, that of instituting a Congress with power to do whatever would be for the good of the United States; and, as they would be the sole judges of the good or evil, it would be also a power to do whatever evil they please.
About this Quotation:
In 1791 Jefferson was very concerned that the creation of a National Bank would open the floodgates for increased spending by the Federal government. As a national bank was not one of the enumerated powers of the federal government in the constitution Jefferson was opposed to its formation, believing that “To take a single step beyond the boundaries thus specially drawn around the powers of Congress, is to take possession of a boundless field of power, no longer susceptible of any definition.” Jefferson appears to make a distinction between the funding of the “Union” and “the general welfare”, where the former meant the operation of the federal government in carrying out its business, and the latter meant an undefined and potentially ever expanding sphere of action which was better left to the individual states or the people. He also raises the classical problem of “qui custodiet custodes” (who will guard our guardians) because with a national bank under its control Congress would then be in a position to both decide what it might do in the name of the general welfare and to give itself the financial means to carry this out, or as Jefferson put it, Congress would have the “power to do whatever would be for the good of the United States; and, as they would be the sole judges of the good or evil, it would be also a power to do whatever evil they please.”