For months, only the most progressive Democrats would dare express their reservations about Hillary Clinton in mixed company. Though polls routinely showed that Democrats wanted to see Clinton face a serious primary challenge — the latest via Bloomberg Politics indicating that 72 percent of self-identified Democrats and independents hope Clinton doesn’t benefit from a coronation – only a handful of Democrats would explain why they hoped she should have to fight for the nomination. That changed earlier this year as prominent liberal politicians began to echo their party’s activist base. Increasingly, Democratic celebrity politicians are openly expressing their fear that Clinton is not progressive enough for her party.

Last week, outgoing Senate Minority Leader Harry Reid joined 5,000 of his fellow Democratic lawmakers and party officials when he signed a Progressive Change Champaign Committee pledge demanding their party’s 2016 nominee pay homage to a variety of far-left policy priorities. The “Ready for Boldness” pledge would compel the Democratic nominee to promise to expand Social Security, break up major financial institutions, and establish “debt-free higher education.”

“Being bold is the only way I’ve ever known how to win,” Reid said in a statement following his decision to endorse this pledge.

“I think there needs to be a vigorous debate in the whole question of running for president,” progressive icon and Bay State Sen. Elizabeth Warren told CBS This Morning hosts last week when asked if she would endorse Clinton when the former secretary of state made her intention to run for the White House public. “I think everyone who is running for president should be talking about what they plan to do to strengthen and rebuild America’s middle class.”

“I’ll tell you where I stand on all the key issues,” she continued. “It’s up to others to say whether they stand there as well, or if they stand in some different place,” she continued, citing her positions on minimum wage, equal pay and trade as examples.”

“I’d like to see her address all of these issues,” Warren said of Clinton before noting that their party is not a “static” thing. “You know, we have a big debate going on right now with trade within the Democratic Party.”

The latest progressive politician to express their reservations in Clinton’s progressive bona fides delivered perhaps the unkindest cut of all. Despite having served as Clinton’s U.S. Senate campaign manager in 2000, New York City Mayor Bill de Blasio refused to endorse Clinton just yet.

“I think like a lot of people in this country I want to see a vision,” De Blasio said of progressives.

“It’s a much different time than 2008,” he added in an interview with NBC’s Chuck Todd. “There are a lot of new issues and new material for Hillary to address.”

Actually, 2016 seems quite a bit like 2008, with the exception that there is no popular alternative to Clinton that unites the left. Though Clinton’s support in polls of Democratic voters remains strong, it is impossible for that to remain the case while the party’s most popular voices refuse to support her candidacy. As Democratic partisans tune into this race, they will begin to look around for an alternative. That alternative’s viability might not matter as much as his or her progressivism.

In the same way that Mitt Romney was never undone by the cast of “also rans,” Clinton will likely survive the challenges she faces from the current slate of Democrats running for the White House in 2016. But while Martin O’Malley, Bernie Sanders, and Lincoln Chafee might be second tier candidates, one or all of them could benefit from an anti-Clinton protest vote. How big that vote will be remains to be seen. But, judging from the current levels of Democratic anxiety, it could be quite significant.