"We are not interested in participating in this political theater", Newsom said at an afternoon news conference in Sacramento.
Providing more rationale in his order for his decision, the governor noted that while sending a surge of USA troops to the border, the federal government has underinvested in federal land management in California even as the state continues to face an unprecedented number of natural disasters - including wildfires - for which the state National Guard serves a critical role. Without California and New Mexico's forces, only just over 1,700 troops would be left in Texas and Arizona.
Newsom is expected to make similar remarks in his first State of the State address on Tuesday.
The Times quoted excerpts from Newsom's State of the State address, which he will give on Tuesday, to illustrate Newsom's hostility to President Trump.
Newsom has been sharply critical of Trump's immigration policies, noting that the president's proposed border wall is unpopular in California and assailing Trump's State of the Union speech for "stoking fear and spreading hatred" by "manufacturing a border crisis". "This is our answer to the White House: No more division, xenophobia, or nativism", Newsom said.
Newsom has long held up California as a more rational alternative to Trump's agenda.
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The expansion of the counterdrug task force requires approval from the US Department of Defense. A budget standoff between Trump and congressional Democrats over the proposal led to a 35-day government shutdown, the longest in US history.
This was in response to the fact that little progress is being made between Democrats and Republicans on the Hill, in terms of reaching an agreement that would help fund Trump's border wall.
"Tolerance for illegal immigration is not compassionate - it is cruel", he said.
Although Trump has ordered thousands of military personnel to be deployed on the southern border, the National Guard remain under the command of authorities in each state. "They will refocus on the real threats facing our state". If Harris and Newsom split the home-state vote, that could provide an opening for other Democrats to get past California's new position in the primary.
Patricia Gandara, co-director of the Civil Rights Project at the University of California at Los Angeles, commended the governor's decision, pointing to the state's own battles over immigration and its treatment of immigrants - including 1994's fight over Proposition 187, which sought to cut off schooling and health services for immigrants here illegally.
"We're still listening, we're still talking, but we're not leading the negotiations", he said.