Louis Plamondon said he was "not surprised" at Annie Lessard's decision to part company with the embattled separatist party.
"Since the past two or three months she has had difficulty because of the vision of the new leader," he said. "I think it's a personality conflict."
Lessard's departure is yet another blow to the struggling party, which was decimated when it lost more than 40 seats in the 2011 federal election.
Since Beaulieu became leader last June, two Bloc MPs have resigned from caucus and the party's only other sitting MP, Claude Patry, has announced he won't seek re-election in 2015.
Other Bloc staffers tendered their resignations soon after Beaulieu's victory, citing his past fiery rhetoric about Quebec independence and nationalism.
Bloc spokesman Simon Charbonneau said Wednesday that Lessard will remain active in the party and stay on as president of her riding association.
"No door was slammed, her departure was friendly," he wrote in an email.
Beaulieu released a statement thanking Lessard for her work and stating he was "happy to have Bloc members of her character at the heart of our party."
Plamondon said Lessard did not support Beaulieu's leadership bid and was close to Daniel Paille, who resigned as leader one year ago due to health problems.
Beaulieu is a hardline separatist and former head of the sovereigntist Societe Saint-Jean-Baptiste.
Former Bloc leader Gilles Duceppe publicly shamed him last June after claiming the new leader's victory speech used words associated to the Quebec terrorist group, the Front de liberation du Quebec.
Beaulieu, who denied Duceppe's claim, vowed after his victory to win back the majority of Quebec's federal ridings in 2015.
One political scientist is predicting Beaulieu's style will likely attract the party core but will alienate softer Bloc supporters, who could consider changing their vote.
Lessard's resignation is just one more blow to the perception among Quebecers that the Bloc is weak and unpopular, said Concordia University professor Harold Chorney.
That, he said, will provide additional incentive for Quebecers to place their strategic votes elsewhere.
Plamondon, who said he believes Beaulieu's leadership style and vision for the party "is the right one," downplayed Lessard's resignation.
"I think it's small news," he said. "It will not make a difference in our determination to continue. It's only an incident, that's all."
Lessard was not immediately available for comment Wednesday.