President Donald Trump explicitly advocated depriving undocumented immigrants of their due-process rights, arguing that people who cross the border into the United States illegally must immediately be deported without trial.
His remarks have sowed more confusion among Republicans ahead of a planned immigration vote this week.
In tweets sent from Virginia, Trump described immigrants as invaders and wrote that US immigration laws are "a mockery" and must be changed to take away trial rights from undocumented migrants.
"We cannot allow all of these people to invade our country," Trump wrote.
"When somebody comes in, we must immediately, with no judges or court cases, bring them back from where they came. Our system is a mockery to good immigration policy and Law and Order. Most children come without parents."
The President continued in a second tweet, "Our Immigration policy, laughed at all over the world, is very unfair to all of those people who have gone through the system legally and are waiting on line for years! Immigration must be based on merit - we need people who will help to Make America Great Again!"
The latest presidential exhortations came as House Republicans were prepping for a vote on comprehensive immigration legislation, after a more hard-line bill failed last week.
Neither bill has Democratic support, and prospects for the second one passing appeared dim, although the White House still supports it.
"I did talk to the White House yesterday. They say the president is still 100 percent behind us," Rep. Michael McCaul, R-Texas, a co-sponsor of the bill, said on "Fox News Sunday."
Some Republican lawmakers are preparing a more narrow immigration bill that would address one of the flaws in Trump's executive order mandating that children and parents not be separated during their detention.
"I think, at minimum, we have to deal with family separation," McCaul said.
The 1997 "Flores settlement" requires that migrant children be released from detention after 20 days, but the new GOP measure would allow for children and their parents to stay together in detention facilities past 20 days.
In the event that the broader immigration bill fails to pass the House this week, the White House is preparing to throw its support behind the narrower Flores fix, which is expected to garner wider support among lawmakers, according to a White House official.
This behind-the-scenes legislative work amounts to a reversal from Trump's position on Friday, when he tweeted that "Republicans should stop wasting their time on Immigration until after we elect more Senators and Congressmen/women in November".
The tweet demoralised Republicans as they headed home for the weekend but did not end talks about what the House might pass.
Marc Short, the White House director of legislative affairs, said on Sunday that it was premature to announce which measures Trump would sign but urged Congress to act quickly to address the immigration issue broadly.
"The White House has consistently raised our concern about the Flores settlement with Congress," Short said. "It's, in fact, an issue that previous administrations grappled with also, and we anticipate Congress acting on that sooner rather than later."
Brendan Buck, counsellor to House Speaker Paul Ryan, (R-Wis), said that a solution specifically dealing with family separation had been "a topic of discussion all week" but that there was not one policy or bill that Republicans had cohered behind.
Meanwhile, Trump's attack on the due-process rights of immigrants follows a week in which he has been fixated on the immigration court system, which he has called "ridiculous". The president has balked at proposals from Senator Ted Cruz, (R-Texas), and other lawmakers to add court personnel to help process more immigration cases.
"I don't want judges," Trump said on Tuesday.
"I want border security. I don't want to try people. I don't want people coming in. Do you know, if a person comes in and puts one foot on our ground, it's essentially, 'Welcome to America, welcome to our country.' You never get them out, because they take their name, they bring the name down, they file it, then they let the person go. They say, 'Show back up to court in one year from now.'"
In his remarks before the National Federation of Independent Businesses, Trump suggested that many immigrants were "cheating" because they were following instructions from their attorneys.
"They have professional lawyers," the President said. "Some are for good, others are do-gooders, and others are bad people. And they tell these people exactly what to say. They say, 'Say the following' - they write it down - 'I am being harmed in my country. My country is extremely dangerous. I fear for my life'. "
Trump said lawyers tell their immigrant clients, "Say that. Congratulations. You'll never be removed."