Xuan-Kha Tran Pham, 49, was taken into custody by Fairfax police following the attack in the offices of Representative Gerry Connolly on Monday.
Attorneys Office in eastern Virginia, which filed the charges, have not said what they believe motivated the attack but said in a written statement that the suspect was demanding to speak to Connolly.
Connolly, a Democrat from Northern Virginia, said on Twitter that the two staff members had been released from hospital.
Prosecutors say Pham entered Connolly's office in Fairfax carrying the baseball bat, then struck a female in the head and hit an intern in the ribs.
He then returned to the first victim, striking her a total of eight times, according to prosecutors.