President Barack Obama is close to announcing major changes to the US immigration system through executive action, US media report.
The overall plan is estimated to affect as many as five million undocumented immigrants living in the US.
Republicans in Congress say such action would be beyond Mr Obama’s authority.
“We’re going to fight the president tooth and nail if he continues down this path,” House Speaker John Boehner told reporters.
Mitch McConnell, the incoming Senate majority leader, urged the president to “work with us to try to find a way to improve our immigration system”.
“The president has been told over and over and over again – and we’re telling him again today – don’t do this because his executive actions are not permanent changes,” he said.
The president also faces pressures from within his own party. Top Senate Democrat Harry Reid urged Mr Obama only to take action after Congress passes a bill funding the government past 11 December.
“It’s up to him,” Mr Reid said. “But I’d like to get the finances of this country out of the way before he does it.”
Some Republicans are pushing for the budget bill to include a statement prohibiting “the use of appropriated funds for the president’s immigration machinations”.
Such a move could provoke a block by the Democrats, or a veto by the president, which in turn raises the risk of a government shutdown.
Unilateral action has been expected on immigration but details of what the president was considering were first reported this week.
At the centre of the reports is a plan to extend the president’s “deferred action” plan, which was designed to protect young adults who were brought to the US illegally as children from being deported.
The plan is to extend that to the parents of children who are US citizens or legal residents.
The action is designed to prevent the break-up of families via deportations. The number of those affected by the suggested policy is based on how long an individual has lived in the US.
If the administration limits the “deferred action” to those who have lived in the US for more than 10 years, it would affect 2.5 million undocumented immigrants, experts estimate.
If the time limit is lowered to five years, it would stop deportations for as many as 3.3 million.
Other parts of the executive action include:
- increasing the number of high-tech workers allowed to live and work in the US
- an expansion of the existing deferred action plans that would move the cut-off date for children arriving to 2010
- shift border security resources to the US southern border, according to reports.
The Senate passed a far-reaching immigration bill in 2013, but the House has not taken up the legislation.
White House press secretary Josh Earnest said the president would not take any executive action if Congress acts.
“If the House does pass the bipartisan Senate bill that’s already passed, the president would happily sign that into law,” Mr Earnest said.
“And if he has already made a decision and moved forward on his own executive actions, he would happily retract those executive actions so that we could implement the bipartisan Senate bill.”