David Axelrod, a former adviser to President Obama, said Tuesday that Hillary ClintonHillary Diane Rodham ClintonDemocrats seek leverage for trial Davis: Trump vs. Clinton impeachments – the major differences Sharice Davids to vote for Trump impeachment articles: 'The facts are uncontested' MORE needs to establish a rationale for her presidential candidacy, saying when he hears "Ready for Hillary," he thinks, "Ready for what?"

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"What happened in 2008 was that Hillary’s candidacy got out in front of any rationale for it, and the danger is that that’s happening again," Axelrod, who was a top adviser to Barack ObamaBarack Hussein ObamaTrump's intervention on military justice system was lawful and proper The mullahs seek to control uncontrolled chaos Poll: Majority of Democrats thinks Obama was better president than Washington MORE's come-from-behind win over Clinton in 2008, said on MSNBC's "Morning Joe."

"You hear Ready for Hillary; it’s like, 'Ready for what?' "

Axelrod said Clinton could be using the time before she announces her candidacy to decide the answer to that question.

"Now Hillary’s task is to define what it is that she’s running for and running about," Axelrod said. "And what would the future look like under another President Clinton, and I suspect that’s what she’s taking her time working through now, but she has to answer that question."

Liberal groups have been pushing forward with an effort to convince Sen. Elizabeth WarrenElizabeth Ann WarrenThe Hill's Morning Report - Sponsored by AdvaMed - House panel expected to approve impeachment articles Thursday Warren, Buttigieg duke it out in sprint to 2020 The Memo: Pelosi-Trump trade deal provokes debate on left MORE (D-Mass.) to challenge Clinton from the left on economic issues.

MoveOn.org, one of the groups, announced Monday that it has passed 100,000 signatures on its "Run Warren Run" petition. Warren insists she is not running but always phrases the denial in the present tense, which some see as leaving the door open. 

Asked if there is an opening for Warren, Axelrod said, "I suppose there is."

"My suspicion is that what she’s doing is trying to influence how Hillary frames her candidacy and the issues she focuses on," Axelrod said. "I think Elizabeth is very sincere about her concerns about what's happening in the American economy, and Hillary hasn’t yet said what exactly her program will be, what she’s running on. I think Elizabeth knows she’s got maximum leverage by still being in the conversation."

Many of Axelrod's fellow Obama campaign alumni are now working for Clinton's campaign-in-waiting in some capacity. Jim Messina, Obama's 2012 campaign manager, for example, signed on as co-chairman of the pro-Clinton super-PAC Priorities USA.

Two more top campaign aides, Mitch Stewart and Jeremy Bird, have joined Ready for Hillary.