President Joe Biden has “limited ability” to step up security at the U.S.-Mexico border via executive action, a top Democratic lawmaker said on Sunday, arguing that the issue should be addressed with legislation in Congress.

Senator Chris Murphy, the lead Democrat who negotiated a bipartisan border security bill introduced this year, said on CBS News’ “Face the Nation” that U.S. courts would likely strike down sweeping action by Biden.

“The president has such limited ability to issue executive orders that would have an impact on the border. He can’t conjure resources out of thin air,” Murphy said. “If he were to try to shut down portions of the border, the courts would throw that out, I think, within a matter of weeks.”

Biden, a Democrat seeking another term in Nov. 5 elections, has said his administration is looking at executive actions to potentially block migrants at the border after Republicans rejected the bipartisan Senate bill this year. Republicans spurned the measure after former President Donald Trump, Biden’s Republican challenger, came out against it.

Senate Republicans blocked the bill again last week and Republican House Speaker Mike Johnson said it “doesn’t secure the border” and “incentivizes further illegal immigration.”

The number of migrants caught illegally crossing the U.S.-Mexico border has dropped this year compared with a monthly record high in December, a trend U.S. officials partly attribute to increased enforcement by Mexico.

Murphy said the decrease was due to “smart, effective diplomacy between the United States and the Mexican government” but warned that the dropoff may not be permanent and that illegal crossings remain high compared with a decade ago.

“We have to just recognize that without updating the laws of this country, without surging more resources to the border, we can’t count on the numbers staying as low as they are today,” Murphy said.

White House spokesperson Karine Jean Pierre said last week that the Senate bill would deliver “significant policy changes, resources, and personnel needed to secure our border and make our country safer.”

Migrants and asylum seekers transit through Mexico to the U.S. to escape violence, economic distress and negative impacts of climate change, according the United Nations.