Donald Trump. AP Photo/Evan Vucci Real-estate mogul Donald Trump escalated his rhetoric on Wednesday against former Secretary of State Hillary Clinton's supposed legal "problem."

Trump, the Republican presidential front-runner, advanced the theory that Clinton has been praising President Barack Obama because she wants to stay out of jail.

"You notice how positive she is about the president? You know why, right? You know why? Because she wants to stay out of the clink. That's why, OK. Believe me, that's why," he told his supporters at a large rally in Tulsa, Oklahoma.

Trump said he was referring to how Clinton's private email setup at the State Department. The FBI has said it is looking into whether any material was mishandled in connection to her account, but authorities have not said that they were looking into Clinton specifically.

Clinton, while saying the decision to use a private email system was a "mistake," vehemently denies she did anything improper. Her campaign has also accused Republicans of selectively leaking information to make her situation look worse than reality.

But Trump said he couldn't help but notice that Clinton has been embracing Obama on the campaign trail. However, that could also be attributed to the fact that she served in his administration and has been facing a stronger-than-expected primary challenge from Sen. Bernie Sanders (I-Vermont).

"I mean, every single thing: 'Oh, the president is wonderful.' She never thought the president was wonderful before. She couldn't stand the president. And frankly he couldn't stand her. Now all of a sudden he's 'wonderful, wonderful,'" Trump said.

"It's very simple," he added. "She wants to make sure she gets through this thing, and he does have the power of the presidency. So who the hell knows?"

Trump made the comments after attacking the media for reporting on Sanders' large crowd sizes even though Trump events are attended by even more people. Trump said he looked forward to running against Sanders, whom he called a "communist" earlier in the rally.

"Bernie will be fun," he said. "I don't know know if Hillary is going to make it."