Watchdog groups and government ethics experts on Wednesday called President Donald Trump's latest tweeted attack on Special Counsel Robert Mueller's investigation —in which he flat-out stated, "Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now"—possibly new evidence that Trump has obstructed justice and a declaration that could potentially be used against the president in the ongoing probe.

After suggesting that anti-Trump text messages sent by FBI agent Peter Strzok provide evidence that the entire investigation was biased against the president, Trump demanded that Attorney General Jeff Sessions shut down the probe.

The president's demand came on the second day of former Trump campaign manager Paul Manafort's trial for charges of money laundering and tax fraud related to work he did for pro-Russia Ukrainian politicians.

In September, another trial is scheduled to begin regarding the nature of Manafort's work in Ukraine.

Manafort—who the president announced was being treated worse than notorious mobster Al Capone—"worked for me for a very short time," Trump wrote in a subsequent tweet.

"Why didn't government tell me that he was under investigation," he added. "These old charges have nothing to do with Collusion—a Hoax!"

The tweeted diatribe amounted to what chair of Citizens for Responsibility and Ethics in Washington (CREW) Norm Eisen called "the latest in a long line of obstructive acts by Trump."

"Efforts to stop an investigation fall squarely within the plain meaning of Sections 1503, 1505, and 1512(c)(2)," wrote the Brookings Institution in its report "Presidential Obstruction of Justice: The Case of Donald J. Trump" last year, referring to the U.S. federal Code of Laws.

Trump's call also did not go unnoticed by the public advocacy group Public Citizen, which issued a reminder that 350,000 Americans have pledged to join nearly 1,000 protests across the country, should the president take direct action to end Mueller's investigation.