President Donald Trump has lashed out at his critics, political rivals, American institutions and even his own national security advisor in a weekend tirade about Russian meddling in the 2016 election.

Yet the president spared Russia itself from his harshest criticism.

Trump zeroed in on his predecessor in the Oval Office, Barack Obama, for not doing enough to stop Russian operations intended to sow chaos in the American political system.

"Obama was President, knew of the threat, and did nothing," Trump posted Sunday morning on Twitter.

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Trump was referring to comments from Rep. Adam Schiff, D-Calif., the ranking Democrat on the House Intelligence Committee, which is conducting its own probe into Russia's election-interference efforts.

Schiff, an outspoken critic of Trump, said Friday that the Obama administration shared some responsibility in the Russia controversy, because it should have been more aggressive and revealed the operation to the public.

"We should have called them out much earlier," Schiff said Friday morning. "While I respect the motive in terms of the Obama administration, they didn't want to be seen as meddling, the American people had a right to know what was going on and could be trusted to do the right thing with it. And they should have defended being more public and aggressive at the time, at least in my view."

Schiff made his comments ahead of several federal indictments of Russian nationals and entities by special counsel Robert Mueller, who is investigating possible collusion between the Trump campaign and the Kremlin.

The indictments revealed Friday did not include any allegation that Russia efforts actually had any effect on the outcome of the election, but it did not conclude otherwise, either. The charges allege that the Russians waged "information warfare" on the U.S. political system to aid Trump's campaign.

The latest developments in the case compelled Trump to acknowledge that Russia had interfered. Previously, he described the Russia plot as a "hoax" and the investigation into it a "witch hunt." In his Sunday tweet storm, he stated that he always meant that the "Russian 'hoax' was that the Trump campaign colluded with Russia - it never did!"

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Trump's tweets fit a pattern for which he is often criticized — that he avoids directly attacking Russia or its president, Vladimir Putin, over what U.S. intelligence and law enforcement agencies have described as widespread Kremlin interference in the campaign.

Earlier this year, the Trump administration decided not to impose additional sanctions against Moscow, which Congress imposed as payback for election meddling.

In another Sunday tweet, Trump blamed American institutions for playing into Russia's hands in its plot to create chaos and division in the U.S. "They are laughing their asses off in Moscow. Get smart America!" Trump tweeted.

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Several Trump appointees, however, have taken a different tack against Russia. Last week, FBI Director Christopher Wray, Director of National Intelligence Dan Coats and other top officials told lawmakers that Russia remained a threat to U.S. elections. "Frankly, the United States is under attack," Coats told the Senate Intelligence Committee.

On Saturday, Trump's national security advisor, H.R. McMaster, explicitly cited the latest indictments as he slammed Russia for its actions in 2016. "As you can see with the FBI indictment, the evidence is now really incontrovertible and available in the public domain," McMaster said at a conference in Germany.

Trump did not appear pleased with his aide's characterization of what happened. Late Saturday night, he tweeted that the Army lieutenant general had "forgot to say that the results of the 2016 election were not impacted or changed by the Russians and that the only Collusion was between Russia and Crooked H [illary Clinton], the DNC [Democratic National Committee] and the Dems."

The latest developments in the Russia probe also colored Trump's thoughts about the school shooting massacre in south Florida last week. In another late-Saturday tweet, the president linked the FBI's failure to follow through on a tip in early January that the alleged shooter posed a deadly threat to the ongoing investigation into ties between his campaign and Russian operatives.

"They are spending too much time trying to prove Russian collusion with the Trump campaign," Trump tweeted.