SACRAMENTO — Federal officials are asking California to send National Guard troops to the Mexican border after President Trump warned of a wave of migrants heading for the U.S. — a request that could set up a new immigration-related conflict between state officials and the administration.

The author of California’s main sanctuary state law, state Sen. Kevin de León, urged Gov. Jerry Brown to reject the federal request. Brown’s office directed questions to the head of the California National Guard, who said the state would answer once it had more details about what Trump wants.

Trump signed a proclamation Wednesday evening to deploy the National Guard, saying he had “no choice but to act” to ensure national security. “With our current laws and resources, we cannot stop illegal aliens from crossing the border,” Trump said.

The administration is asking California, Arizona, New Mexico and Texas to send Guard troops immediately. Homeland Security Secretary Kirstjen Nielsen said federal officials would work with the governors of those states and that she hoped the first troops would arrive Wednesday night.

The request came after Trump took to Twitter over the past few days to vent his frustrations with Congress’ refusal to fund a border wall and warned about a “big Caravan of People from Honduras, now coming across Mexico and heading to our ‘Weak Laws’ Border.”

Trump was referring to a group of about 1,200 migrants traveling north from Central America. The springtime influx of migrants happens every year, and many of them seek protection in Mexico rather than attempting to cross into the U.S.

De León, whose SB54 sanctuary law minimizing local law enforcement’s cooperation with federal immigration agents is the target of a Trump administration lawsuit, said Brown “should ignore Trump’s pointless and expensive call.”

“Despite Mr. Trump’s rantings, there is no evidence of imminent influx of asylum seekers here in California or elsewhere,” said de León, D-Los Angeles, who is challenging U.S. Sen. Dianne Feinstein in the June primary. “In fact, border crossings have plummeted in the last decade and are at the lowest level since 1971.”

A report issued this month by the U.S. Customs and Border Protection agency said the number of unaccompanied migrant children detained along the 2,000-mile Mexican border was down 24 percent from the previous year. The number of detained families dropped 32 percent, the report said.

Under the federal-state setup of the National Guard, governors are the commanders-in-chief of their state’s forces. The president has the option to federalize Guard forces, but it has happened only rarely.

Brown’s office deferred comment on Trump’s request to Lt. Col. Tom Keegan, spokesman for the California National Guard. He said it would be “promptly reviewed to determine how best we can assist our federal partners.”

“We look forward to more detail, including funding, duration and end state,” Keegan said.

Nielsen said in Washington that deployment details were still “being finalized.”

The governors of the other three border states covered by Trump’s request are all Republicans, and all indicated they would comply.

The California National Guard has 55 soldiers working directly with border enforcement on an anti-drug mission, Keegan said. The federal government last asked California for additional staffing at the border in 2006 and 2010.

In 2006, Gov. Arnold Schwarzenegger sent 1,000 additional National Guard troops to the border in response to a request by President George W. Bush, but only after a protracted dispute over who would pay for it.

Schwarzenegger sent 224 National Guard troops to the Southwest border in 2010 at President Barack Obama’s request.