Jefferson's whole political philosophy was based on a radical distrust of central government power and a correspondingly narrow interpretation of that power under the terms of the Constitution. The Louisiana Purchase seemed to go against Jefferson's long-standing republican principles. For one thing, the Constitution did not authorize the federal government to buy property from foreign governments, yet that's precisely what the Purchase involved. Jefferson's Federalist opponents immediately seized on what appeared to be the president's hypocrisy and made an issue out of it.
Jefferson didn't want to be seen as playing fast and loose with the Constitution, but at the same time, he didn't want to lose this once-in-a-lifetime opportunity to expand American territory at what was a fairly reasonable cost. He sent the Louisiana Purchase to the Senate—without amendments—for ratification. Republican control of the Senate ensured that the measure passed comfortably after only a two-day debate—Jefferson's enormous constitutional gamble had paid off.