(Carolyn Kaster/AP Photo)

Hillary Clinton has remained coy on her potential presidential candidacy in 2016, but that hasn't stopped Democrats from endorsing her. The Ready For Hillary Super PAC has been in full swing organizing Clinton supporters, and in January 60 congressmen said they would endorse her if she were to run.

Some major Democrats have spoken out against endorsing Clinton too early, including some who have indicated they may challenge her for the nomination. Here are six prominent Dems who aren't ready for Hillary.

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1. Massachusetts Gov. Deval Patrick

He may share a hometown with Clinton (Chicago), but any Windy City solidarity hasn't yet been extended to the realm of presidential politics. When asked in May on CNN's "State of the Union" if Clinton's potential candidacy would lead to an inevitable victory, he said, "I guess I worry a little bit. She's an enormously capable candidate and leader. But I do worry about the inevitability thing, because I think it's off-putting to the average…voter."

2. Ohio Sen. Sherrod Brown

The senior senator from one of the most important election states is already tired of all the 2016 press. When asked by U.S. News last month about endorsing Clinton, he said, "I'm not on board with anybody…It just doesn't matter to me at this point. I'm not trying to be arrogant about it, I've just got more important things to do. It'll matter to me sometime next year at some point, I guess." He also chose to stay out of the 2008 Democratic primary, only endorsing Obama after he had secured the party's nomination.

3. Iowa Sen. Tom Harkin

A common criticism from Republicans about Clinton centers on the ambiguity of her core principles. But some on the left feel the same way, including Harkin. In June, Harkin told the Des Moines Register: "I think Hillary would be the first to say no one ever has something all locked up. This is not a coronation or anything like that." He added, "Democrats would rightfully say anywhere, 'Wait a minute. No matter who is running, we want to know who, why, what do you stand for? What would your policies be as president?' That is especially true in Iowa."

4. Maryland Gov. Martin O'Malley

O'Malley has positioned himself as one of Clinton's first prospective opponents, which explains why he would not be eager to endorse her this early. Regarding the recent controversy over immigration, he split with Clinton when speaking at a National Governors Association meeting earlier this month, saying, "We are not a country that should turn children away and send them back to certain death." O'Malley has also been at odds with the Obama administration over their handling of the immigration crisis, leading to a public spat between him and the White House.

5. Bill Richardson, former governor of New Mexico and cabinet member

Richardson has been a vocal critic of Clinton in the past, and he doesn't seem ready to change his thinking two presidential election cycles later. When asked by Larry King last week on King's show "Politicking" if he thinks Clinton has the nomination in hand, Richardson stayed distant: "I want to see who the candidates are. I think there should be open competition," he said. "It's not that I won't ever be there, but right now I'm not one of those hundreds of Democrats flocking and saying the race is over."

6. Brian Schweitzer, former Montana governor

Schweitzer didn't mince words when asked about Hillary Clinton's presidential aspirations, positioning himself as one of Clinton's loudest critics on the left. In June he told Time, "You can't be the candidate that shakes down more money on Wall Street than anybody since, I don't know, Woodrow Wilson, and be the populist. You can't be the one to say we're going to focus on rebuilding America if you voted to go to the Iraq War." When asked if he thinks he would be a better fit in the Oval Office, Schweitzer was adamant: "Well, I think so, of course."

ABC News' Caleb Jackson contributed to this report.

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