Why did Jefferson's critics attack him on the Louisiana Purchase?
The short answer to this is that Jefferson's critics attacked him out of political opportunism. But according to Jefferson's own political philosophy, one which he had espoused since he was Secretary of State, they had a fair point. Jefferson's critics, most of whom were Federalists, argued that the Constitution did not explicitly permit Congress to purchase land. To defend the Purchase was to argue for a loose interpretation of the Constitution, an approach that Jefferson had argued against on a number of issues in his career, most notably the chartering of the Bank of the United States. Indeed, Jefferson's so-called "strict constructionism" was the cornerstone of the Republican faction for which he was the spokesperson. The Federalists, whose political base was in the eastern port cities, also knew that the opening of western territories would speed up westward expansion, a process which would doom their party. Modern historians usually interpret Jefferson's decision as an example of the ways Presidents are often forced to abandon strict adherence to ideological principles when confronted with the realities of running the country.