[2][3] In 2014, she was elected to the Michigan House of Representatives as a Republican from the 42nd district, succeeding Bill Rogers, who was ineligible to run for reelection due to term limits.

In January 2021, Theis was one of 11 Republican Michigan state senators who promoted Trump's false claims of fraud in the 2020 election; in a letter sent to Congress on January 6, 2021, ahead of the 2021 United States Electoral College vote count, Theis and the other members of the group baselessly suggested that there were "credible allegations of election-related concerns surrounding fraud and irregularities.

[12] In June 2021, the Republican-controlled Michigan Senate Oversight Committee (of which Theis was the vice chairwoman) issued a report finding no support for Trump's claims of fraud.

[5] In 2022, Theis wrote in a campaign fundraising email that State Senator Mallory McMorrow and other Democrats wanted to "groom and sexualize kindergartners".

[13] In 2022, Theis was reelected to the state Senate after she was challenged in the Republican primary election by Mike Detmer, a far-right conspiracy theorist endorsed by Trump.