Conservative critics of Ted Cruz are going after his tithing practices. According to recently released tax records, the Texas senator contributed less than 1 percent of his income to charity between 2006 and 2010. But many Christians believe that the Bible commands a charitable offering, or tithe, equal to 10 percent of one’s annual earnings.

This discrepancy could end up making a difference less than two weeks before the caucuses in Iowa, a state where a Republican politician’s faith matters. And this is exactly what a newly formed political group, Americans United for Values, is hoping for. Today, the group is launching a 60-second radio advertisement on news, talk, and Christian stations across Iowa that raises the tithing question and labels Cruz a “phony”: “He doesn’t tithe?” a female voice asks in the ad. “Isn’t he a millionaire? His wife worked for a big Wall Street bank, right?”

While a candidate’s charitable giving might seem petty or irrelevant to nonreligious Americans, it likely matters to the conservative Christians who Cruz has worked diligently to court. A 2011 study by Pew Research Center found that 61 percent of American evangelical leaders deemed tithing essential to being a “good evangelical.” And an additional 34 percent said it is “important” though not “essential.” If religious voters see Cruz’s meager tithing as an indication that he isn’t as faithful as he presents himself to be, he could lose their support. And since Cruz and Trump are in a dead heat in Iowa, even a small shift could be devastating.

Republican rival and former Southern Baptist minister Mike Huckabee for one was more than happy to point out Cruz’s apparent insincerity. “It’s hard to say God is first in your life if he’s last in your budget,” Huckabee told BuzzFeed News. “If I say I’m a vegan but you look at me eating hamburgers and rib eye every night, you’re going to say, ‘I don’t think this guy’s really a vegan.’”

Which is why, if the tithing accusations do stick and cost Cruz the nomination, the Texas senator will have been the author of his own demise. His near-constant emphasis on religion can be seen in the arc of his campaign: Cruz announced his run for president at Jerry Falwell’s Liberty University. His website includes a registration page for supporters to join a “National Prayer Team.” He worked hard to orchestrate the perfect family-prayer scene for a television ad. And he won the collective endorsement of religious right leaders just last month.

The entire architecture of his campaign is based on constant appeals to believers, invoking strong references to his faith at campaign stops and promising to protect “Christians who have been persecuted for their beliefs.” He even calls himself a “Christian first, American second.” And in New Hampshire, where religion plays a smaller role in politics, Cruz recently declared that America should return to its Judeo-Christian roots: “For too long there has been a spirit of fear and timidity in Washington. We should not be ashamed of Christ.”

That’s why charges that Cruz doesn’t practice what he preaches strike right at the heart of his most loyal voter base. And it seems perfectly fair that Bible-believing Christians would hold Cruz to the standard he himself set. After all, it was Jesus who preached, “In the same way you judge others, you will be judged, and with the measure you use, it will be measured to you.”

Many Christians also consider tithing to be about far more than mere money. It’s an indication of how devoted one is to God. Again, as Jesus said, “For where your treasure is, there your heart will be also.” Cruz’s treasure has stayed firmly with Cruz. So for Christians, this tithing revelation calls into question not just Cruz’s generosity but his level of commitment to one of God’s commands and to his fellow man.

Cruz has energized religious voters by preaching Christian values at campaign stops across the United States. But since Cruz has exalted himself as the most faithful candidate in the Republican field, evidence challenging that claim casts doubt on his image and trustworthiness among Christian voters. For many believers, it’s disingenuous to preach Christian values with your mouth and then deny them with your wallet.