From cupcakes to calf-roping to a miracle cure, Sid Miller’s colourful career as Texas’s agriculture commissioner has taken on a darker hue with the news that he is the latest high-ranking official in the state to face a criminal investigation.

The Texas Rangers are examining claims that Miller misused taxpayer funds by flying to Oklahoma to receive the “Jesus shot”, reportedly a $300 injection said to cure chronic pain for life. The injection is only available from a physician known as “Dr Mike”, whose medical license in Ohio was revoked after he was convicted of multiple felonies including healthcare fraud and tax evasion.

The Rangers have also been asked to consider allegations that Miller spent about $2,000 in state and campaign funds to fly to Mississippi and compete in a rodeo. News of the trips – both in February last year, a month after Miller took office – was revealed by the Houston Chronicle, which said that Miller spent two days competing in calf-roping contests at the Dixie National Rodeo and won $880.

The advocacy group Progress Texas filed a criminal complaint last month requesting that the Rangers investigate Miller for the Oklahoma visit.

This week the group asked the Rangers to look at the rodeo trip as well. On Wednesday the Rangers said they have started a criminal investigation. “The Texas Rangers are conducting a criminal investigation on allegations that Texas agriculture commissioner Sidney Miller violated state statutes regarding abuse of official capacity. Once the investigation is completed, it will be referred to the Travis County district attorney’s office,” said a Texas department of public safety spokesman, Tom Vinger.

“All the facts point to Sid Miller breaking the law,” said Lucy Stein, the Progress Texas advocacy director. “As a steward of the people’s money, Sid Miller should be held accountable for abusing his office. We are hopeful that the Texas Rangers will conduct a thorough and impartial investigation into Miller’s criminal conduct.”

An agriculture department representative did not return a request for comment. Miller told the Chronicle that the complaints had no merit and are “harassment by a very left-leaning liberal group”.

The newspaper said that Miller used a departmental credit card for the flights to Mississippi, later paying back the state using money from his campaign account after, he said, a work meeting with an institute that promotes farm-raised catfish failed to materialise.

Miller decided to refund the cost of his Oklahoma visit last month after it emerged that despite its billing as a business trip he failed to show up to a meeting with one official and seemingly met others more or less in passing. It is against Texas law to use state funds for personal reasons.

Rarely seen without a cowboy hat, Miller is a rancher and expert calf-roper who has won nine American Quarter Horse Association championship titles. The 60-year-old Republican was elected agriculture commissioner in November 2014, running for the job on a platform that included tightening border security, protecting the second amendment and defunding Planned Parenthood.

He is a former state representative. In 2011 he authored a bill that forces women seeking abortions to undergo an ultrasound and have the fetus described to them, and a “pork chopper” law allowing Texans to shoot feral hogs from helicopters. Last year a Miller campaign Facebook page shared a post that appears to advocate using nuclear warfare on “the Muslim world”.

His first press conference as commissioner in January last year was held to tout the recent reversal of a longstanding ban on sugary and fatty foods in Texas public schools – a rule, he said, “that sounds like something from the Obama administration”.

Munching on a strawberry cupcake, he declared he was “also granting full pardon to pies and cookies and brownies and cakes and homemade candies” and wanted to lift restrictions on deep-fat frying and sodas in schools.

A 2011 study classified nearly one-fifth of children between ages 10 and 17 in Texas as obese, one of the worst rates in the country.

Miller’s communications director, Lucy Nashed, resigned on Monday because of a “tremendous lack of communication” at the department, she said in a statement.

She previously worked as a spokeswoman for former Texas governor Rick Perry. His short-lived second tilt at the Republican presidential nomination was complicated by felony abuse-of-power charges related to an unsuccessful attempt to force out the Democratic district attorney for Travis County, Rosemary Lehmberg, following her drunk-driving arrest in 2013. Perry was cleared of all charges in February this year.

The inquiry into Miller follows news on Monday that the federal Securities and Exchange Commission has filed a civil fraud lawsuit against the Texas attorney general, Ken Paxton. The Republican was indicted in 2015 on criminal charges also related to allegations of improper conduct regarding investor recruitment in a tech company before he became the state’s top lawyer. He denies wrongdoing. In 2014, during his election campaign, he was fined by the Texas State Securities Board for soliciting investment clients without being properly registered.

“There’s certainly a history in the state of politicians being accused of scandals and hanging on, both for Democrats and Republicans,” said Brandon Rottinghaus, a political scientist at the University of Houston who has studied the effects of political scandals.

Rottinghaus said that senior officials often survive crises while lower-ranking staff members fall on their swords. As with Perry and Paxton, he said, the lack of public criticism of Miller from colleagues so far suggests there is no appetite within the party to force his removal, especially early in the legal process and with no imminent campaign to fight in a Republican-dominated state where GOP candidates are routinely elected to key positions.