Dismayed by the Trump administration's first days, former President Barack Obama's loyalists, former aides and even his spokesman are speaking out and even actively resisting the new American leader. It's a warning to President Donald Trump that his actions won't go unchallenged by those who occupied the White House before him.

While it doesn't appear that anyone is coordinating the flurry of tweets, public statements and direct challenges to Trump, former Obama administration officials said they're taking cues from their longtime boss. In his final weeks, Obama pledged to be an unobtrusive and deferential ex-president, but pointedly reserved the right to protest if Trump violated what Obama considered core American values.

It took Trump barely a week to cross Obama's threshold, with an executive order cracking down on refugee admissions and a ban on travel from seven majority-Muslim countries. Obama, on a post-presidential vacation in California, broke his silence this week through a spokesman. Obama "fundamentally disagrees" with religious discrimination, the spokesman said.

Obama's loyalists already were weighing in.

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His U.N. ambassador, Samantha Power, tweeted the day Trump was inaugurated: "Raise your hand if you're ready to defend everything we have built together these last 8 years — at home and abroad!" A week later Obama's national security adviser, Susan Rice, called Trump's restructuring of the National Security Council "stone cold crazy."

Yet Obama, who polls showed had left office popular and trusted, had appeared more cautious about diluting his influence by quickly second-guessing Trump. Doing so could make it easier for Trump to dismiss critiques as predictable partisan nitpicking. And becoming the face of Trump's opposition could make it harder for the next generation of Democratic leaders to emerge.

So Obama said nothing Monday when Trump fired his former appointee Sally Yates, who was serving as acting attorney general. He also won't opine on Trump's announcement of a Supreme Court nominee, former White House aides in touch with Obama said.

Following his lead, Team Obama is applying what former aides call a "Mitt Romney-John McCain" test to Trump. If Trump's actions seem like something the former GOP presidential nominees might have done, Obama won't criticize. Actions outside the Republican mainstream are fair game.

Dan Pfeiffer, Obama's senior White House adviser, and three other former aides have launched a podcast called "Pod Save America," a kind of public group therapy session for despondent Obama loyalists. Their latest episode decried "Trump's unconscionable Muslim ban" and discussed Trump's Supreme Court deliberations with a former White House lawyer. The Trump administration says the travel order doesn't target Muslims, and focuses on terrorism-prone countries.

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"The perspective we bring is as people who have sat in those offices and understand the seriousness and the decisions that cross the president's desk, and we are deeply concerned about what is happening," Pfeiffer said in an interview.

Denunciations reached a fever pitch after Trump's order on refugees and travel to the U.S by people from seven Muslim-majority countries. Obama's former national security spokesman, Ned Price, said the Trump administration's claim that it only affected a minority of travelers was "absurd and un-American."

Price added: "Nearly every racist & xenophobic movement in history could be said 2 target a 'minor portion' of a population."

When Trump's White House claimed Obama, too, temporarily banned Iraqi refugees, the ex-president's deputy national security adviser Ben Rhodes shot back: "This is a lie."

Obama loyalists said the expressions of opposition aren't part of an organized campaign. They reported being energized by a series of group text messages, Facebook groups and email chains in which some of the thousands of one-time Obama staffers are sharing their dismay.

One notably silent figure: Former Vice President Joe Biden, who has yet to criticize Trump since leaving office. An aide said Biden planned to speak out on matters he cares most about, such as foreign policy and women's issues. The aide wasn't authorized to comment publicly and requested anonymity.

A few who served in Obama's administration have registered objections formally.

Hundreds of diplomats signed a memo to State Department leadership this week criticizing Trump's immigration order, defying a White House warning that they should "get with the program" or resign. The move was striking because the diplomatic corps is comprised mostly of career diplomats who serve presidents of both parties.

It was Yates, Obama's deputy attorney general, who mounted the boldest act of resistance. She directed the Justice Department to stop defending Trump's immigration order in court.

Trump had elevated Yates to acting attorney general until his own nominee could be confirmed. After her protest, Trump quickly fired her and called her weak on immigration.

Obama's own attorney general, Loretta Lynch, rushed to Yates' defense. She called the decision to defy Trump "courageous."