Campaign finance law prohibits direct communication about spending between a campaign and supportive super PACs, but campaigns and political committees now try to send signals about spending through different channels. And the specific language that Halle used ("it's critical they see this on the air") has become more commonplace among political strategists seeking to stretch the limits on coordination without breaking the law — though they typically aren’t broadcast on Twitter.

Buttigieg swore off any super PACs formed specifically to back him, but he has benefited from more than $1.3 million in New Hampshire TV ads from VoteVets, a veterans’ group that backs Democratic candidates and endorsed Buttigieg in December. The ads feature veterans who support Buttigieg.

Buttigieg’s campaign did not deny that the tweet was a hint to his backers.

“Pete is the only candidate who isn’t a millionaire or billionaire. And if the largest progressive veterans group wants to help spread the word about his service, we welcome it,” said Chris Meagher, a Buttigieg spokesman, in a statement shared with POLITICO. “Veterans have been central to propelling our movement and they will be a central part of the coalition that Pete will bring together to defeat Donald Trump.”

VoteVets, for its part, said it doesn't take cues from the campaign.

"VoteVets PAC is running an independent expenditure campaign. We independently decide our ad strategy,” said Jon Soltz, VoteVets chairman, in a statement. “We cannot and do not coordinate our ads with the campaign in any way.”

But Halle’s tweet threatened to reignite a fierce battle over campaign finance that Buttigieg and Elizabeth Warren have been fighting for months. Warren and Bernie Sanders have sworn off the kind of high-dollar, in-person fundraising that Buttigieg has embraced in this campaign, culminating in Warren’s infamous debate-night shot at Buttigieg’s fundraiser in a California wine cave.

“Was this meant to be a [direct message] or did you mean to tweet out this instruction to your super PAC?” tweeted Roger Lau, Warren’s campaign manager, quoting Halle’s own post. “Fun fact about how some campaigns exploit our broken campaign finance laws: if it was a DM it would be illegal.”

Throughout the fall and early winter, the pair got consumed by a tit-for-tat argument over transparency. Buttigieg called on Warren to release her pre-2008 tax returns, while Warren criticized Buttigieg for raising money at private, high-dollar fundraisers. Buttigieg ultimately opened his fundraisers to a pooled reporter.
Joe Biden also has help from a supportive super PAC, Unite the Country, which spent millions of dollars on TV advertising in Iowa before the caucuses.

One Democratic strategist called Halle’s tweet a “textbook” way “to target a super PAC,” while another Democratic operative noted that “it happens all the time.”

“This is very clearly a smoke signal to an independent spender, which while curious to look at, is very legal,” said the Democrat, who has worked with independent expenditures in the past and granted anonymity to discuss the issue candidly. “People are going to try to sensationalize this, but it happens all the time in a variety of ways.”

But Buttigieg’s opponents largely aren’t claiming his campaign is acting illegally; they’re just outraged that the conduct is legal. The Progressive Change Campaign Committee – another outside group that has endorsed Warren, but is not spending any money on TV ads on her behalf – put out a statement blasting the Buttigieg campaign.

"Pete entered the race as a Boy Scout but has corrupted his brand by becoming the candidate of big-money corporate donors,” said Adam Green, co-founder of the PCCC. “It's a slap in the face of campaign finance law to so brazenly and unethically direct a Super PAC how to spend on his behalf -- all while leaving New Hampshire to do big-money New York fundraisers. Donald Trump would call him part of the corrupt swamp if he were ever the nominee."