Officials with End Citizens United said its fund-raising and promotional activities for the presidential candidates would begin immediately.

Tiffany Muller, the president of the group, said getting involved in the presidential race was an essential next step. Calling President Trump “the most corrupt president in American history,” she said the seven candidates who had taken End Citizen United’s 2020 pledge had shown a distinctive commitment to the group’s agenda.

“Committing to ‘Reform First’ shows the American people you’re serious about cleaning up corruption,” Ms. Muller said. “These candidates uniquely understand that reform is the essential first step to accomplishing the progressive change that America needs.”

For many of the candidates who signed on to the End Citizens United pledge, ethics and government reform have long been central campaign issues. Ms. Warren, for instance, promised months ago, in a February interview on MSNBC, that her first major legislation as president would be an anti-corruption bill. Ms. Gillibrand unveiled a plan in May to overhaul campaign finance by giving every voter $600 that they could donate to candidates. And Mr. Bullock entered the Democratic race with a video that highlighted his record of fighting undisclosed “dark money” campaign contributions in Montana.

The “Reform First” pledge came together quietly over a period of months, as End Citizens United strategists met with more than a dozen campaigns to lay out the parameters and argue that it was a politically advantageous space to occupy in the primary. In a PowerPoint presentation reviewed by The New York Times, the group made the case that both Mr. Trump and former President Barack Obama won their elections in part because they were perceived as tougher on corruption than their opponents.

In 2020, the End Citizens United presentation predicted, “the candidate trusted to change the rigged system will win.”