Mike Folmer was detained on Tuesday, after Tumblr, the social media site, tipped off authorities about the Pennsylvania state senator’s online behaviour.
Detectives searched his home and allegedly found two indecent images of minors on his mobile phone.
Tumblr approached the National Center for Missing and Exploited Children in February.
The site told officials that a user had uploaded at least one image containing alleged child sexual abuse in December 2017.
Detectives then traced the account to Mr Folmer’s email address and home.
The 63-year-old was charged on Tuesday with possession of the images and criminal use of a communication facility.
“Michael Folmer stated that he had been dealing with some personal problems/issues and that he had received child pornography through his Tumblr blog,” an agent from the Pennsylvania attorney general’s office wrote in court papers.
“I will continue to say it - no one is above the law, no matter what position of power they hold,” Josh Shapiro, Pennsylvania’s attorney general, said in a statement.
“I will continue to work to protect children and hold those who abuse them accountable.”
The 63-year-old did not respond to requests for comment but his fellow state Republicans plan to strip him of his chairmanship of the senate state government committee.
The politician was elected to a fourth term in the Pennsylvania senate in November 2018.
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He is considered one of the most conservative members of the chamber, known for regularly pulling a miniature copy of the US Constitution from his suit pocket.
Mr Folmer had recently opposed legislation which would have helped victims of historical child sexual abuse sue institutions such as the Catholic Church.
His bail was set at $25,000 (£20,000) but it is unclear if it has been paid.
Additional reporting by agencies