"Without the phony Russia Witch Hunt, and with all that we have accomplished in the last almost two years (Tax & Regulation Cuts, Judge’s, Military, Vets, etc.) my approval rating would be at 75% rather than the 50% just reported by Rasmussen," Trump tweeted.

Trump blasted Mueller's probe as "Presidential Harassment!"

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The president's approval in the Rasmussen poll is higher than most other surveys. Gallup's tracking poll showed Trump's overall approval this week stands at 40 percent.

Trump's message is the latest example of him venting frustration over the increasing pace of developments in the Russia investigation.

Some legal observers have speculated the developments signal the investigation is moving closer to Trump's inner circle.