COUNCIL BLUFFS, Iowa — The Democratic National Committee has opened the door to allowing former Mayor Michael R. Bloomberg of New York to participate in the presidential debate it will sponsor in Nevada next month, a change from its practice that demanded candidates demonstrate grass-roots fund-raising support.
The D.N.C. announced Friday that in order to participate in the debate, set for Feb. 19 in Las Vegas, a candidate must win at least a single delegate in either the Iowa caucuses or the New Hampshire primary or meet polling requirements.
It has eliminated the requirement that candidates must have received donations from hundreds of thousands of individuals. Mr. Bloomberg, a multibillionaire, is running a self-funded campaign and is not soliciting donations.
The changes, which represent the most significant tightening of debate requirements this cycle, set off a fresh and pointed round of criticism at a critical moment in the race, as several campaigns braced for the reality check that the Iowa caucuses will provide. And the edict from party officials, which some saw as a concession to Mr. Bloomberg, quickly reignited concerns among those who believe the D.N.C.’s shifting rules for the debates privilege some candidates and campaigns over others.
To meet the latest polling threshold, candidates must earn 10 percent in four qualifying national polls or 12 percent in two polls taken in Nevada or South Carolina by a qualifying pollster.
Mr. Bloomberg, who is not competing in the first four nominating contests, held in those two states, Iowa and New Hampshire, has notched 10 percent in one national poll, which was released by Fox News on Jan. 26. He has until Feb. 18 to perform similarly well in three other national polls.
We are thrilled that voters could soon have the chance to see Mike Bloomberg on the debate stage, hear his vision for the country, and see why he is the strongest candidate to defeat Donald Trump and bring our country together,” Kevin Sheekey, Mr. Bloomberg’s campaign manager, said in a statement.
He added, “Mike has run for office three times and never taken a dime from special interests, allowing him to act independently, on the merits, without having to do what donors expect. He is proud to be doing the same with this campaign.”
“To now change the rules in the middle of the game to accommodate Mike Bloomberg, who is trying to buy his way into the Democratic nomination, is wrong,” said Jeff Weaver, a senior adviser to Mr. Sanders. “That’s the definition of a rigged system.”
Former Mayor Pete Buttigieg of South Bend, Ind., Senator Amy Klobuchar of Minnesota, the entrepreneur Andrew Yang and the hedge fund billionaire Tom Steyer, each of whom qualified for next week’s debate in New Hampshire, could still make the one that follows in Nevada by reaching the polling threshold or winning a delegate in the Iowa caucuses or the New Hampshire primary.
If they don’t, it would suggest their campaigns faced significant obstacles to continuing.
Mr. Buttigieg, speaking in Council Bluffs, said he was fine with the new requirements.
“When the initial rules came out with a focus on grass-roots fund-raising, of course our campaign was focused on the same thing,” he told reporters. “Now it sounds like there is a different focus and I do believe that we will qualify.”
Tom Perez, the chairman of the D.N.C., signaled in December that he would be willing to alter the debate criteria to allow Mr. Bloomberg to participate once primaries and caucuses began.
But he also said then: “Our logic for having a grass-roots fund-raising threshold was, it provides opportunity for people who don’t have the national name I.D. to get on the debate stage. If you want to win the presidency, you’ve got to connect with grass-roots America.”
Xochitl Hinojosa, a D.N.C. spokeswoman, argued Friday that the first two nominating contests would adequately measure the grass-roots enthusiasm around a particular candidate, and that the donor threshold was therefore unnecessary.
“Now that we will have the results of two elections and grass-roots support is actually captured in voting, that will replace the donor threshold,” she said.
During the fall and winter, as the D.N.C. began making it harder to qualify for the debates, some candidates who found themselves short of the thresholds began agitating for more inclusive rules.
Senator Cory Booker of New Jersey, for example, aired concerns about the diversity of the group that had qualified for the winter debates and led an effort to persuade Mr. Perez to make changes that would expand the roster. Mr. Booker dropped out of the race after failing to qualify for the debates held in December and January.
And as it became increasingly likely that he would not qualify for the debate held earlier this month in Iowa, Mr. Yang wrote a letter to Mr. Perez suggesting that the D.N.C. commission more qualifying polls.
In a statement posted on Twitter on Friday, S.Y. Lee, a spokesman for Mr. Yang, criticized the rule change, arguing it would help Mr. Bloomberg.
“It’s a mistake for @TheDemocrats to change the rules for debates in the middle of this race to yield to a billionaire,” Mr. Lee said. “We need to respect the grass roots movement leading this party forward.”
Former members of the Booker campaign underscored just how much of a financial burden the individual donor threshold had placed on campaigns over the summer. Addisu Demissie, who was Mr. Booker’s campaign manager, said on Twitter that one of the most significant days of his campaign was when the donor threshold doubled from 65,000 to 130,000.
For a lot of the smaller campaigns, doubling their donor counts was essentially an expense, as buying email lists and running acquisition ads on Facebook replaced other campaign priorities such as digital ads and staffing in early voting states. Jenna Lowenstein, a deputy campaign manager for Mr. Booker, said on Twitter that they had thrown out an entire strategic plan for those states in order to buy email addresses.
Mr. Steyer, who argued in December for allowing Mr. Booker and other candidates who missed the polling thresholds onto the debate stage, lamented Friday’s rules change.
“Changing the rules now to accommodate Mike Bloomberg and not changing them in the past to ensure a more diverse debate stage is just plain wrong,” he said. “They are changing the rules for a candidate who is ignoring early states’ voters and grass roots donors.”
Reid J. Epstein reported from Council Bluffs, and Matt Stevens from New York. Nick Corasaniti contributed reporting.