Senate Judiciary Committee Chairman Lindsey Graham warned Democrats they should “think twice” about inviting former special counsel Robert Mueller to testify again about his nearly two-year Russia investigation.

Mueller’s mental acuity, which came into question last year following halting testimony about his report on Russian interference in the 2016 election and possible obstruction of justice by President Trump, became a topic of discussion during a Thursday debate on granting Graham subpoena power in his Crossfire Hurricane investigation.