Sally Yates warns Trump's 'all-out assault on the rule of law' has already targeted Bob Mueller
Sally Yates (MSNBC)

Sally Yates said President Donald Trump had moved dangerously close to a red line by ordering an investigation into his predecessor.

The former acting attorney general, who was fired by Trump after issuing a warning about then-national security adviser Mike Flynn, appeared Monday on MSNBC's "Morning Joe" to discuss the president's order -- which deputy attorney general Rod Rosenstein has so far agreed to indulge.

"What we're seeing here is that the president has just taken his all-out assault on the rule of law to a new level, and this time he is ordering up an investigation of investigators who are examining his own campaign," Yates said. "You know, that's really shocking."

Yates agreed the president's order could put Rosenstein or FBI director Christopher Wray in a position where they're forced to resign, but she said his ongoing assaults on law and order were already dangerous enough.

"One of the things we have to look out for is the old 'boiling the frog slowly' issue," Yates said. "You know, we become accustomed to things that the president does, in directing DOJ. I can remember a time when he would issue a tweet or directive and the reports would be, 'in an unprecedented act,' the president did X or Y. It's not so unprecedented anymore and oftentimes it doesn't even make it through the full 24-hour news cycle."

She said the president did not have to order the firing of special counsel Robert Mueller to impede the investigation into his campaign ties to Russia -- or any of the other various crimes his associates have been accused of or pleaded guilty to committing.

"I think we all know, and happily folks on both sides of the aisle have recognized, that firing Bob Mueller would be red line," Yates said, "but we also need to recognize that firing Rod Rosenstein would be equally damaging to the rule of law because Rod Rosenstein here really controls the scope of the special counsel's investigation and trying to get rid of him is having a back doorway of trying to control that investigation."