Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE's strong support from K Street poses a tough dilemma for his campaign.

The influence world is stocked with former aides and supporters who have rallied around his previous bids for president. In this cycle, though, those lobbyist ties, past fundraising from corporate interests and perceptions that Biden is more favorable to businesses could hurt his bid for the Democratic nomination.

Biden has quickly solidified his Democratic front-runner status and focused his attention on President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE.

His campaign has said he will not take money from lobbyists and corporate PACs, but that is unlikely to be enough for progressive groups in the primary who have larger concerns about the candidate.

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“With Joe Biden, if he wants to say no to corporate lobbyists' money that’s great and it’s a step in a positive direction that acknowledges the times,” Adam Green, co-founder of the Progressive Change Campaign Committee, told The Hill.

“But, with Joe Biden, it’s not about course correcting any one little thing, it’s about his big picture brand, which is being cozy with big corporations and cutting back room deals with Republican political insiders.”

Biden's allies run deep on K Street, where a number of former aides from his time as a senator now hold high-level positions at powerful lobbying firms.

Christopher Putala, who founded the lobbying firm Putala Strategies, was a lawyer on the Senate Judiciary Committee for Biden, as was Jeffrey Peck, now a lobbyist at Peck Madigan Jones.

Biden also has allies in Tony Russo, a lobbyist at T-Mobile, who served as his legislative counsel in the Senate; Larry Rasky, the chair of Rasky Partners, who worked on Biden’s 1988 and 2008 presidential campaigns; and Ankit Desai, a political assistant to Biden in the Senate and now a lobbyist at Tellurian.

And Biden's more than three decades in the Senate and previous runs for president will give his critics plenty of fodder.

When Biden ran for president in 2008, he raised money from lobbyists. He reversed course when he joined the ticket with President Obama, who made running against K Street and rejecting corporate money a centerpiece of his first presidential campaign.

In the Senate, Biden also represented Delaware, a state that is home to many large corporations, including a number of credit card giants.

Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.), one of his rivals for the 2020 nomination, took a shot at Biden last week, accusing him of being on the side of "the biggest financial institutions" over "hardworking families."

This year, Biden also held a fundraiser hosted by David Cohen, telecom giant Comcast's chief lobbyist. And Biden allies led by Democratic fundraiser Matt Tompkins quickly launched the For the People PAC after he officially jumped into the race, a move first reported by The Hill. The PAC aimed to raise millions to boost Biden's bid.

His campaign, though, was quick to distance itself from the super PAC, telling The Hill that "Vice President Biden does not welcome assistance from super PACs."

Republicans, who see Biden as a strong challenger to President Trump, have also called for more scrutiny over the business dealings of his son Hunter Biden and potential conflicts of interest.

As vice president, Biden pressed Ukraine to dismiss a prosecutor, who faced accusations he had ignored corruption among officials in the government. The prosecutor was eventually removed.

The New York Times in a story this week reported that Hunter Biden was on the board of an energy company the dismissed prosecutor was investigating. Trump's personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE on Thursday called for an investigation into "Biden conflicts" of interest.

Biden's campaign told the Times that his son's business dealings had no connection to policies Biden carried out as vice president.

The issue of corporate ties has taken newfound importance in the Democratic Party, where liberal groups are pressing candidates to reject special interest cash.

“There’s a new benchmark of what Democratic campaigns are now judged by, a new litmus test, and it would be hard for any candidate to not reject [lobbyists’ money],” Zach Friend, a Democratic strategist and former spokesperson for Obama for America, told The Hill.

“It’s how you enter into the race. It would be equivalent to any other Democratic policy — do you support unions? Do you support marriage equality? Do you support choice?”

The scrutiny on Democrats is intense.

Pete Buttigieg Pete ButtigiegThe Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE, the mayor of South Bend, Ind., who has seen his stock rise in polls of the Democratic primary race, has found strong support on K Street, especially among LGBTQ lobbyists who are rallying behind the openly gay 2020 contender. But that support led Buttigieg last week to say he would no longer accept lobbyist donations and that he would return the $30,000 he received in the first quarter of the year.

Not taking lobbyist money poses its own challenges for Biden, and he will need to show his strength at raising small-donor donations, as Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) has, to stay competitive.

Biden's allies, though, won't be on the sidelines.

Those on K Street noted there are other ways for lobbyists to help without writing a check.

“There are plenty of ways to help,” Al Mottur, Democratic lobbyist at Brownstein Hyatt Farber Schreck, told The Hill. Often, Mottur said, lobbyists can help a candidate by introducing them to other big donors.

“But if you can’t give or bundle that is a big prohibition,” he acknowledged.

“How do you get introduced to a bundler in San Francisco when you’ve never considered California a relevant state before? They might not be giving money but they may be introducing it,” Friend added.

Mottur said lobbyists can also "help with strategy" or “provide issue area support in your areas of expertise.”

And they can be influential surrogates, pushing the candidate's message in the media and among other important political actors.

But Biden will need to walk a fine line. Progressive groups say they will be watching Biden, and the other candidates, closely over their ties to special interests.

Lisa Gilbert, vice president of legislative affairs at Public Citizen, said despite past relationships with lobbyists, candidates should be judged on what they do now.

“The effort to keep corporate money out of politics is now pretty much the expected route for candidates," Gilbert told The Hill.

"We think that’s a phenomenal thing and something that we aren’t surprised that candidates who hadn’t done that before are choosing to do so."