Since launching his presidential bid, former Vice President Joe Biden has largely settled on a primary strategy common among frontrunners: Only attack when attacked, stay above the conflict, and let the other candidates squabble among themselves so they can eventually take one another out.

That last tactic is why Biden hasn’t been going after Pete Buttigieg. He doesn’t think the South Bend mayor can hurt him, but he does think Buttigieg can hurt someone else who could—Sen. Elizabeth Warren.

Biden’s campaign declined to comment on the record about its strategy. But multiple veteran pollsters and operatives said there’s sound logic behind the notion that the vice president’s team might view Buttigieg as a useful way to help blunt Warren’s momentum, particularly with white college-educated voters. The more Buttigieg rises in states like Iowa and New Hampshire, the thinking goes, the more Warren slips in some polls.

“There’s not a reason to believe Buttigieg at this point is really hurting Biden,” Monmouth University pollster Patrick Murray told The Daily Beast. “The more the waters in Iowa are muddied, the better it is for Biden.”

One well-placed Iowa Democrat put it more candidly, “You don’t need to punch down at the mayor of South Bend, Indiana.”

One exception to that rule occurred on Monday, as Biden cruised on his extended bus tour through the Hawkeye State, a place where Buttigieg has recently gained ground and Warren has an equally strong following with just nine weeks to go before the first votes are cast.

Responding to a reporter’s question about whether he unintentionally set the mayor up for success by providing a blueprint for a moderate health-care plan, which features similar contours to his own, Biden said: “Set it up? He stole it! Set it up?”

The Buttigieg campaign sees it differently, with one official telling The Daily Beast that “Pete has been talking about his health-care plans since the day he launched his exploratory committee in January and was talking about ‘Medicare for All Who Want It’ months before Biden entered the race.”

The two rivals are indeed offering voters a similar vision to address their medical needs. Biden’s campaign wants to expand the Affordable Care Act, the signature Obama-era health-care law, by adding a “public option” that would allow the choice of a government plan in addition to private insurance. Buttigieg is also campaigning on a public option, using the phrase “Medicare for All Who Want It” as a twist on the “Medicare for All” proposals favored by more liberal Democrats, including Sen. Bernie Sanders and Warren.

But the unscripted dig at Buttigieg was noticed, in part, because it was a rarity in much of Biden’s months-long campaign.

“I doubt that’s made it into the stump speech,” Peter Leo, the Democratic chair in Carroll County , Iowa, said about the comment. “You don’t look good as the former VP treating the mayor of South Bend like he’s a threat to your candidacy.”

Fred Yang, a Democratic pollster with Hart Research, said it “makes a lot of sense” for Biden’s team to lay low on Buttigieg, particularly as some polling indicates his rise in key areas comes with Warren’s decline.

“It would seem odd for the [former] vice president to be attacking Pete Buttigieg right now,” Yang said.

The South Bend mayor’s ascent in Iowa is more alarming for Team Biden when contrasted with Warren’s numbers in the state, where she has been considered a frontrunner for months after investing vast resources there. In the latest Iowa State University survey, Buttigieg shot up 13 points in two months, earning 26 percent in November, compared to 13 percent in September. In the same poll, Warren went from 24 percent to 19 percent during the same time. Warren’s campaign did not immediately respond to a request for comment.

The rising tension between Warren and Buttigieg spilled out into full view on Thursday night, when the Massachusetts senator took the rare step of criticizing a Democratic rival by name, suggesting Buttigieg hasn’t been transparent about aspects of his closed-door fundraisers.

“Mayor Pete should open up the doors so that anyone can come in and report on what’s being said,” Warren said at a gaggle in Boston. “Those doors shouldn’t be closed and no one should be left to wonder what kind of promises are being made to the people that then pony up big bucks to be in the room.”

Minutes later, Buttigieg’s senior adviser, Lis Smith, shot back: “If @ewarren wants to have a debate about transparency, she can start by opening up the doors to the decades of tax returns she’s hiding from her work as a corporate lawyer-often defending the types of corporate bad actors she now denounces.”

That type of dynamic⁠—both campaigns spontaneously sparring⁠—is a best-case scenario for Biden, strategists said, and something his campaign has watched play out over the past several weeks in other forms, dating back to Buttigieg’s critique of Warren on health care during the Democratic debate in Westerville, Ohio, in October.

Still, there are places where Buttigieg won’t be as useful in helping slow Warren’s rise, including in states with more diverse populations than Iowa and New Hampshire, where the South Bend mayor has struggled to catch on.

Throughout much of the primary cycle, Warren was inching up on Biden’s lead in pockets of the country, exhibiting potential at times to dent the strong support he enjoys from a sizable and reliable base of African-American voters. In nine Quinnipiac University polls conducted between March and October, for example, Warren was steadily gaining support with black Democrats, an Axios analysis found. In November, a group of more than 100 black female activists endorsed her presidential bid.

But Biden still has an overwhelming base of black supporters who have remained loyal to his campaign. Buttigieg, in contrast, has struggled consistently to make headway. Given the former vice president’s lead with that voting bloc, and Buttigieg’s failure to catch on so far with the same constituency, it would be tactically unwise for Biden to go after the mayor, strategists said.

There’s also the multitask factor. Jabbing multiple contenders in the same political party can appear less than dignified, a principle Biden has based much of his candidacy around upholding, often saying on the stump that he’s waging a “battle for the soul of this nation” against President Trump. And he’s already been sharply attacking Warren as his main target for months.

On top of that, there’s the ideological overlap. With similar moderate positioning, it’s harder to draw natural distinctions between Biden and Buttigieg, who are each making “electability” arguments that beating the current White House occupant requires a more middle-of-the-road approach than what Warren or Sanders are offering.

“If you’re Biden, maybe they’re thinking they’ve lost ground to Warren more so than Buttigieg,” a separate longtime Democratic strategist who has worked on several presidential campaigns said.

In March, 23 percent of voters found Warren to be too liberal, according to a Des Moines Register/CNN poll. In November, that number rose to 38 percent, with only 7 percent of voters putting Biden or Buttigieg in the too liberal category, according to the November survey.

“They are waiting to see if Buttigieg’s electability argument undermines their own,” Murray said about Biden.

For progressive Democrats, the strategy also appears to be more closely tied to ideology, with multiple strategists pointing to the similarities between Biden and Buttigieg on campaign finance reform in particular. While both candidates are holding high-ticket fundraisers, Warren has sworn off of them, making it harder to distinguish between the two contenders and opening up the line of contrast she tried out Thursday night.

“It is a dog fight up top,” Murshed Zaheed, a former leadership staffer for Harry Reid, said. Pointing to Buttigieg and Biden’s welcoming of corporate sources to help fund their campaigns, he speculated they likely see Warren as the most credible threat.

“I think that’s really why these guys are buddying up.”