Those brief letters by Mr. McDonald — the complaint against Mr. Paxton alleging he violated state securities law was a single page — turned the key in the Texas court system that led to the involvement of prosecutors, judges and grand juries. Filing complaints directly with prosecutors is not unique to Texas: An official with the National District Attorneys Association said most states allowed local prosecutors to initiate cases based on citizen complaints and allegations.

“We don’t set out to turn our elected officials into criminals,” said Mr. McDonald, the former national field organizer for Public Citizen, the advocacy group founded by Ralph Nader. “We merely, in both of these cases, plus the Tom DeLay case, thought we witnessed a crime and we called the cops. And we think as a watchdog that’s our role.”

The indictments of the state’s then-governor and sitting attorney general, 11 months apart, have put the group in the spotlight and in the cross hairs of critics. Mr. Perry and Mr. Paxton are Republicans in a Republican-dominated state. After the grand jury indicted Mr. Perry last August, Texans for Public Justice temporarily shut down its Facebook page because of hostile comments and removed its list of financial supporters from its website “to protect them from harassment,” Mr. McDonald said. One woman posted a photograph online with an aerial view and map to Mr. McDonald’s home. For weeks, he felt more comfortable locking the doors of his office while he and his colleagues worked inside.

“I hope that one day, the good people of Texas will have heard enough of your vile hate-speak and destroy your organization to the ground,” one man wrote in an email to the group.

Mr. McDonald says his complaints have not been politically motivated, and represent a small portion of the group’s work, most of which focuses on collecting and analyzing Texas campaign finance data. Mr. McDonald, who describes Mr. Nader as his mentor, takes his work seriously but finds time for his hobby: running Lost Art Records, the two-artist record label he co-owns that features live recordings of deceased singer-songwriters from Texas.