President jefferson asserted a lot of executive power in order to purchase the louisiana territory from the french. how did he and his political allies justify that?

Jefferson and those favoring the Louisiana Purchase justified it as an act done for the good of the country.

Initially, President Jefferson had commissioned James Monroe and Robert Livingston to negotiate a deal with France to acquire New Orleans or all or part of Florida, as a means of avoiding the potential of an armed conflict in such areas.  Monroe and Livingston were authorized to spend up to $10 million.  What they found out was that Napoleon was already set to sell a much wider range of territory to the United States, to finance his European wars.  Napoleon was asking $22 million for the whole territory that became the Louisiana Purchase.  The US team negotiated the price down to $15 million.  But then there was a constitutional crisis back home.  Did the President have the authority under the constitution to make such a major addition to the nation's territory and spend the nation's funds to do so?  Jefferson himself considered pursuing a constitutional amendment, but his Cabinet members disagreed and the measure was sent to Congress for approval.  In a statement he made at the time, Jefferson justified the purchase with this analogy:  "“It is the case of a guardian, investing the money of his ward in purchasing an important adjacent territory; and saying to him when of age, I did this for your good."