WASHINGTON — President Donald Trump’s administration is preparing a request for as much as $6.6 billion for a down payment on his promises to build a wall at the U.S.-Mexico border and crack down on undocumented immigrants.

The supplemental request may be sent to Congress as early as next week, according to people familiar with the plan. That would jump-start a debate in Congress over the president’s far-reaching proposals, the costs, and which undocumented immigrants to target for deportation.

Trump made the border wall a centerpiece of his political campaign for the presidency, regularly telling crowds of supporters that the U.S. would build a “great” structure and have Mexico cover the cost.

An early version of the plan would include $2.8 billion to begin carrying out Trump’s executive orders calling for construction of a border wall and increased deportation of undocumented immigrants, the people said. About half of that amount would be used for a major increase in detention facilities and for alternatives including ankle bracelets.

About $1 billion would be set aside for construction of the border wall. Other funding includes $115 million for added Immigration and Customs Enforcement personnel and another $115 million for Customs and Border Protection hiring.