Republican presidential candidate Donald Trump attends a news conference near the U.S.-Mexico border (background), outside Laredo, Texas. Reuters/Rick Wilking

The president reiterated a variation of one of his key campaign promises on Sunday, tweeting that Mexico would pay for the wall Trump wants to build across the border between the southern United States and Mexico.

"Eventually, but at a later date so we can get started early, Mexico will be paying, in some form, for the badly needed border wall," Trump tweeted.

Trump also attacked Democrats in Congress who did not want to allocate US funding towards constructing the wall.

"The Democrats don't want money from budget going to border wall despite the fact that it will stop drugs and very bad MS 13 gang members," he tweeted.

Trump kicked off his presidential campaign in June 2015 by saying that Mexico would immediately pay for the border wall and repeated that throughout the election cycle. Shortly before the election, after Mexican president Peña Nieto said multiple times that his country would not pay for the wall, Trump softened his stance and said that Mexico would "reimburse" the US for the cost of the wall.

Congressional leaders have said that Congress will pay for the cost of the wall by this fall.

Congress will pay for "the construction of the physical barrier on the border," House Speaker Paul Ryan said in January.

"We intend to address the wall issue ourselves," Senate Majority leader Mitch McConnell said that month.

Trump has frequently criticized the media for documenting his shift on how the wall will be funded:

The president's stance on the wall was echoed Sunday by his attorney general, Jeff Sessions, on ABC's "This Week."

"We're going to get it paid for one way or another," Sessions said.