Senators Cory Booker (N.J.) and Kirsten Gillibrand (N.Y.), who top the list of potential Democratic challengers to President Donald Trump in 2020, both announced last week that they would forgo accepting donations from corporate PACs in an effort to cure the corrosive effect they say special interest spending has had on the political system. But neither extended their pledge to labor unions and ideological groups.

Gillibrand made her pledge last week. It was quickly followed by Booker, who, echoing Gillibrand, said the "campaign finance system is broken."

Neither's pledge to stymie outside spending, however, extended to labor unions and ideological groups that have long been supportive of Democratic causes and candidates. Under the law, corporate PACs and PACs associated with labor unions and ideological groups operate under the same standard.

Each organization raises funds through a network of employees and individuals who voluntarily contribute donations, which in turn are used to support candidates who express ideological and policy positions that align with the priorities of the PAC's membership. PACs affiliated with corporations, labor unions, and ideological groups are prohibited from directly coordinating with candidates and political parties, but they can raise unlimited funds and are under no legal obligation to disclose their donors. Each organization is also limited to a direct monetary contribution of $5,000 per candidate, per election cycle.

One such group, the American Association for Justice, which represents the interests of thousands of trial lawyers across the country, has donated heavily to both Gillibrand and Booker throughout their congressional tenures.

Booker and Gillibrand's decisions were applauded by End Citizens United, a left-wing political advocacy group explicitly working to overturn the 2010 Supreme Court decision in Citizens United v. Federal Elections Commission. The ruling prohibited the government from restricting the amount of money independent expenditure groups could spend in support of or against specific candidates. Overturning Citizens United has become a rallying cry for the left with liberal firebrands such as Sens. Elizabeth Warren (D., Mass.) and Bernie Sanders (I., Vt.) lambasting it as one of the worst Supreme Court decisions in history.

"Senator Gillibrand has built her career on the promise to lead by example…. She has seen firsthand how corporate mega-donors manipulate Congress and put politicians in their pocket to pass bills, like the disastrous tax bill. By taking the pledge, Senator Gillibrand is taking a firm stand against Big Money," Tiffany Muller, president of End Citizens United, said in a released statement.

Muller also praised Booker as demonstrating "leadership" in reforming a "broken system so everyone has a voice in our democracy."

"Senator Booker's decision to reject corporate PAC money demonstrates his leadership in the fight to unrig the system that's leaving too many Americans behind. He's showing he will be accountable to the people and not the agenda of corporate special interests," Muller said. "End Citizens United thanks Senator Booker and looks forward to working with him to reform the broken system so everyone has a voice in our democracy."

In a twist of irony, End Citizens United was one of the largest outside groups spending money in support of Democratic senatorial candidates in Nevada and New Hampshire during the 2016 election cycle. The organization's PAC spent over $2 million in each state, which amounted to the 10th and 11th largest outside independent expenditures for those races.

It is unclear why Gillibrand and Booker decided now was the right time to stop taking business PAC donations. It is also unclear why both senators have decided not to extend the ban to labor unions and ideological groups. Neither office responded to requests for comment on this story.

Gillibrand was first elected to Congress in 2006 and has accepted corporate PAC money throughout her tenure in Congress. Records compiled by the Center for Responsive Politics show that between 2005-2018, Gillibrand raised $4.9 million from business PACs, which constituted only 9.7 percent of her total fundraising, but over 65 percent of the total contributed from political action groups. In that same time period, Gillibrand raised nearly $1.1 million, 14.3 percent of her total PAC donations, from labor unions and over $1.5 million, around 20 percent, from ideological groups.

In her video announcement last week, Gillibrand elaborated that the Citizens United decision weighed heavily on her decision. The senator, however, continued to accept corporate PAC donations in her two most recent Senate elections, in 2010 and 2012, which occurred after the ruling.

Booker, whose congressional tenure began in 2013, has raised $1.8 million from business PACs composing 8 percent of his total fundraising, but 76 percent of the total his campaign has raised from political action groups. Between 2013 and 2018, Booker raised $279,600 from labor unions, accounting for over 11 percent of his total PAC contributions, and $287,432, around 12 percent, from ideological groups.

In the past, Booker has been accused by the base of the Democratic Party of being too close to Wall Street. Detractors cite votes in support of pharmaceutical companies and the fact that Booker received more money from the securities industry than any other candidate, Republican or Democrat, on the ballot in 2014—even though he was only facing nominal opposition—as proof of pro-business sentiment.

Neither Booker nor Gillibrand have announced plans to return the corporate PAC donations their campaigns have collected. Neither senator has announced any intention to reject the support of super PACs, which can raise and spend unlimited funds on behalf of candidates for elective office. Super PACs, which are required to disclose their donors, can also accept unlimited funds from groups like corporate PACs, labor unions, and trade associations with ideological leanings.

Furthermore, neither Booker nor Gillibrand has refused to stop taking individual donations from employees of corporate organizations, which make up a significantly larger portion of their total fundraising numbers. From 2005 to 2018, Gillibrand raised only $35,498 from the corporate PAC affiliated with pharmaceutical giant Pfizer, Inc., but contributions from individuals employed by Pfizer accounted for $106,000 for the senator's campaign. The same goes for Booker, who raised $12,500 from the business PAC of Prudential Financial, one of the nation's largest life insurance companies, while individuals employed by the company donated $59,000.