Thursday night may mark the first debate in which Donald Trump and Sen. Ted Cruz go after each other, thank God. As such, it will be the first opportunity to see how the weaponry they’ve already been testing out against one another performs in a face-to-face setting. There’s reason to believe that Trump has the advantage here. Though Cruz might think he’s played Trump better than Trump has played him thus far, Trump has been getting the better of his coattail-rider in the past couple of weeks.

The latest Des Moines Register/Bloomberg poll from “gold standard” Iowa pollster J. Ann Selzer showed Trump cutting into Cruz’s lead from the previous survey. Selzer’s mid-December poll showed Cruz with a 10-point margin over Trump, 31 percent to 21 percent; the one released Wednesday has Cruz at 25 percent to Trump’s 22 percent. Several other polls this week have shown Trump with a slight lead in Iowa, giving Trump a modest edge over Cruz in the RealClearPolitics polling average.

What effect could the Cruz birther business, injected into the conversation early last week by Trump, have here? Selzer’s survey, like the Huffington Post/YouGov findings earlier this week, doesn’t see widespread Republican concern about it—yet—but does find enough concern that it might make a difference in a tight race. Fifteen percent of respondents in that poll are “bothered” by Cruz’s Canadian birth while 32 percent of Trump’s supporters are. “Fifteen percent in Iowa,” Selzer tells Bloomberg, “when you have this many candidates, if that’s part of what’s taking people to Trump, it’s not nothing.” Bloomberg’s Joshua Green adds that it doesn’t help Cruz to have all of his lesser rivals for the same niche of hard-right Iowa support—like Sen. Rand Paul, Mike Huckabee, and Rick Santorum—echoing Trump’s questioning.

If the birther thing bops Cruz out of the first-place Iowa finish that’s come to be expected of him, it will be because of the brilliant way Trump went about business here. Rather than outright saying that Cruz is a shady, maple-tree–hugging Canadian treason baby, and thus risking serious blowback from conservatives, Trump played the just sayin’, this is going to be a big problem if the Democrats bring it up in court card. That way, those who might be torn between Cruz and Trump could be persuaded, as unlikely as it seems, to consider Trump the safer political bet. As one likely Iowa caucusgoer told a Weekly Standard reporter: “I liked Ted Cruz as well. But now there’s questions about his citizenship. I don’t want to wait and see what’s going to happen with that.”

Trump’s comments, and the subsequent validation from an august figure like Harvard law professor Laurence Tribe, have also sparked something of a debate among legal scholars on the issue. Though the consensus still holds that Cruz is eligible, Cruz can’t turn for help to his friends in the Republican leadership, because he has no friends in the Republican leadership.

The birther attacks have been concerning enough to stir Cruz from his no-tackle policy against Trump. Speaking on the Howie Carr radio show Tuesday, Cruz said that “Donald comes from New York and he embodies New York values.”

Trump’s response, when asked about which dastardly New York values he embodies, was perfect. “One thing it means is energy. You know, when the World Trade Center got hit, we rebuilt that World Trade Center and we got through and very few places in this world could have gotten through what we went through,” he said Wednesday. “I mean, I was so proud of New York, the World Trade Center, these two massive, 110-story buildings come down, thousands of people killed. I’ve never seen anything like it in my life. And immediately the rebuilding started.” Again, Trump didn’t respond in the negative personal way that he’s roped Cruz into doing. He simply pointed out that Cruz is saying these things because “he’s very nervous. He’s, ya know, very frightened by what’s happening I guess.” This explanation has the added political benefit of being true.

There’s more for Trump to work with. The New York Times published a story Wednesday night about how Cruz, despite saying that he and his wife funded his 2012 Senate campaign by scraping together their personal savings, also took out and didn’t properly disclose two low-interest loans from Goldman Sachs and Citibank.

The story itself isn’t a Cruz death sentence, but it does offer rivals an on ramp into piercing Cruz’s image as some sort of outsider, hayseed, simple country lawyer taking on fancy, big city interests left and right. Cruz is a Princeton- and Harvard-trained corporate lawyer whose wife is on leave as a Goldman executive, and whose well-funded presidential campaign has drawn fantastic sums of lucre from members of the New York financial industry. As I wrote in December, Trump is the candidate best positioned to launch these criticisms against Cruz as another bought-and-paid-for stooge of special interests, since Trump’s strongest cross-ideological appeal is that he (theoretically) can’t be bought.

We’re seeing evidence that Trump is getting under the usually unflappable Cruz’s skin. Surveying the graveyard of fallen rivals who’ve tried and failed to combat Trump thus far, it’s clear that doesn’t end well.

Read more of Slate’s coverage of the GOP primary.