Donald Trump said on Sunday he would allow the federal government to shut down if Democrats refuse to back major changes to immigration laws.

“I would be willing to ‘shut down’ government if the Democrats do not give us the votes for Border Security,” the president tweeted, “which includes the Wall! Must get rid of Lottery, Catch & Release etc. and finally go to system of Immigration based on MERIT! We need great people coming into our Country!”

The federal government has already shut down twice this year, first over a failed deal for Dreamers, young undocumented migrants brought to the country as children, then over a funding bill.

Following a day of relative silence on Twitter on Saturday, Trump’s shutdown tweet was part of an early Sunday broadside on immigration. The president made reference to the administration’s controversial and cancelled policy of separating migrant families at the southern border.

Trump’s tweet came several days after the government said more than 1,800 children separated at the US-Mexico border have been reunited with parents and sponsors after a federal judge ordered the reunions. Hundreds of children remain separated.

Republicans have made immigration a central plank of their appeal to voters ahead of the November midterm elections, in which polls suggest significant Democratic gains are likely.

“Please understand,” Trump wrote, “there are consequences when people cross our Border illegally, whether they have children or not – and many are just using children for their own sinister purposes.

“Congress must act on fixing the DUMBEST & WORST immigration laws anywhere in the world! Vote ‘R’”.