President Trump is increasingly optimistic about the outcome of special counsel Robert Mueller's investigation into possible collusion between the GOP candidate's campaign and Russia, according to a report published Monday.

However, some of Trump's allies are worried he is not taking the probe seriously and should brace himself for a bad ending, according to three dozen sources who spoke with CNN.

"The president's mantra is 'All this Russia stuff, it's all going to wrap up soon.' He repeats it as fact," one person said. "Part of me is like — 'Are you serious? You believe this?'"

Trump has appeared less stressed behind the scenes, even telling friends and advisers that he thinks Mueller will clear him of wrongdoing by the end of January.

Trump has said Mueller would soon write a letter that declares him innocent. Mueller's team has made no indication it will vindicate Trump or others on his staff in coming weeks.

This week, Trump's private lawyers will meet with Mueller as part of standard investigation procedures.

Trump told reporters over the weekend he has no plans to fire Mueller after recent speculation.