There's a specific reason why Trump leveled an unprecedented level of frustration toward the Russia probe this weekend

• This past weekend, President Donald Trump fired off 14 tweets following the latest charges in the Russia investigation.
• Trump alternated between declaring his innocence, blaming former President Barack Obama for Russia's election interference, accusing Hillary Clinton of collusion, and suggesting the FBI would have caught the Florida school shooter if it hadn't been so focused on the Russia probe.
• The charges the special counsel announced Friday do not implicate any members of the Trump campaign or the White House.
• But Trump's unprecedented level of public frustration with the investigation likely stems from his belief that Friday's indictment threatens the legitimacy of his election victory.

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It was a busy weekend for President Donald Trump.

From his Mar-a-Lago Club, 36 miles from the school where a deadly shooting claimed 17 lives last week, Trump spent the bulk of the long weekend weighing in on the Russia investigation via Twitter.

His tweets came in slow succession after special counsel Robert Mueller's office announced charges on Friday against 13 Russian nationals and three Russian entities for carrying out a social media disinformation campaign aimed at sowing discord to influence the 2016 US election.

It's not a new practice for Trump, who uses Twitter for everything from excoriating his critics to announcing new policies (at times, without his advisers' knowledge).

But this Presidents' Day weekend, the commander-in-chief fired off an unprecedented 14 tweets about the Russia probe - declaring his innocence, suggesting the FBI would have caught the Florida school shooter if it hadn't been working on the investigation, blaming Democrats and former President Barack Obama for Russia's interference in the election, and accusing former Democratic presidential candidate Hillary Clinton of colluding with Russia.

Friday's charges were the fifth set released in Mueller's investigation, which is examining the extent of Russia's interference in the race and whether members of the Trump campaign colluded with Moscow to tilt it in his favor. The indictment did not contain allegations that the Trump campaign was knowingly involved in Russia's scheme. It also did not make a judgment on whether the Russian defendants' actions altered the outcome of the election.

Despite that, last weekend marked the first time since former FBI director James Comey confirmed the investigation's existence in March 2017 that Trump had been so vocal about his criticisms of the Russia probe.

The reason, experts say, is that even without specifically naming Trump or his associates, Mueller effectively threw a wrench into Trump's most passionate belief about his ascendance to the Oval Office: that he got there on his own merit and without any outside help.

'But wasn't I a great candidate?'

Trump has handed out copies of the 2016 Electoral College map showcasing his victory to reporters in the Oval Office, and reportedly has a framed copy hanging in the hallway.

"I did what was an almost an impossible thing to do for a Republican-easily won the Electoral College!" he tweeted last April. He later added a claim that experts immediately discredited: "I won the popular vote if you deduct the millions of people who voted illegally."

Of Russia's involvement in hacking the Democratic National Committee, Trump said during the campaign, "I mean, it could be Russia, but it could also be China. It could also be lots of other people. It also could be somebody sitting on their bed that weighs 400 pounds."

When the CIA determined in late 2016 that Russia put its thumb on the scale during the election in a specific effort to aid the Trump campaign, his transition team released the following statement: "These are the same people that said Saddam Hussein had weapons of mass destruction. The election ended a long time ago in one of the biggest Electoral College victories in history. It's now time to move on and 'Make America Great Again.'"

When Trump finally acknowledged in early January 2017 that Russia meddled in the election, he tacked on an addendum. "Hacking's bad, and it shouldn't be done. But look at the things that were hacked, look at what was learned from that hacking." He also didn't say the Kremlin's actions were aimed at boosting his candidacy.

In the tweetstorm on Sunday morning, Trump Rep. Adam Schiff, who is the ranking Democrat on the House Intelligence Committee, was investigating Russian collusion because Democratic presidential nominee Hillary Clinton lost to Trump.

"Now that Adam Schiff is starting to blame President Obama for Russian meddling in the election, he is probably doing so as yet another excuse that the Democrats, lead by their fearless leader, Crooked Hillary Clinton, lost the 2016 election," the president tweeted. "But wasn't I a great candidate?"

'This undermines the legitimacy of his election - and even his presidency'

Meanwhile, Mueller's indictment on Friday laid out in minute detail the elaborate campaign that Russian actors mounted to sway American voters in favor of Trump.

The indictment "hits close to home" for Trump, said Jeffrey Cramer, a longtime former federal prosecutor and the managing director at Berkeley Research Group.

It speaks solely to the issue of Russia's involvement in influencing the election, he said, adding, "this president takes personally any allegation that he won for any reason other than his decisions on the campaign trail."

The Internet Research Agency, the notorious Russian "troll farm" named as a defendant in the indictment, began its operation to interfere in the 2016 election as early as 2014. At the time, Trump had not declared his candidacy but had indicated his interest in a presidential run to multiple news outlets.

As the campaign season got underway, the defendants in the indictment engaged in a social media influence operation meant to denigrate Clinton and strong Republican contenders like Ted Cruz and Marco Rubio.

Trolls were instructed to use any opportunity to criticize candidates across the field "except [Bernie] Sanders and Trump" because "we support them."

In late summer 2016, Russia-linked Facebook accounts posing as American activists also reached out to at least three Trump campaign officials to ask if they could assist the campaign's operation in Florida.

By that point, the court filing said, the defendants' "operations included supporting the presidential campaign of then-candidate Donald J. Trump … and disparaging Hillary Clinton."

Jens David Ohlin, a vice dean at Cornell Law School and an expert on criminal law, said Monday that Trump appears to be "enraged" by Friday's indictment because it is the first time the Russia investigation has produced evidence in such specific detail "that Russian actors interfered in the election and covertly supported Trump."

He added: "For Trump, this undermines the legitimacy of his election - and even his presidency."