And they're blocking emergency funding to Ukraine in order to get it.
A record number of migrants have been caught illegally crossing since Democratic President Joe Biden took office in 2021 - there were about 2 million in fiscal year 2023.
Far more than the around 850,000 seen in 2019 during Donald Trump's presidency.
Here's what Republicans and Democrats want to do.
Republican U.S. House Speaker Mike Johnson has said Biden needs to reinstate Trump policies and crack down on border crossings.
JOHNSON: "If President Biden wants a supplemental spending bill focused on national security, it better begin by defending America's national security."
Republicans also oppose new Biden policies that allow certain migrants to enter legally for humanitarian reasons.
Republicans have called on the Democratic-led Senate to approve a bill passed by the House in May that would effectively end access to asylum at the border.
The White House has said Biden would veto the bill.
A White House funding request pairs tens of billions of dollars for Ukraine and Israel with nearly $14 billion for border security.
That money would pay for new U.S. Border Patrol agents and asylum officers.
The package has been blocked by Republicans in the Senate - who say it doesn't do enough to discourage illegal immigration.
A bipartisan group of senators has been talking for months about a possible deal, but so far haven't agreed to anything.
Lawmakers must pass spending bills by January 19 and February 2 to avoid a partial U.S. government shutdown.
Some Republicans say any spending legislation must include their border bill or other language that will prevent migrants from being released into the U.S.
It's unclear whether they will force a shutdown over the issue.
