The Greens have backed Labor over cutting cash handouts for non-taxpaying shareholders, despite previously saying it would hurt vulnerable seniors.
Labor sought to wedge the Greens over their "contradictory" stance on the policy in the Senate on Wednesday.
Greens finance spokeswoman Sarah Hanson-Young initially welcomed the policy last week, only to be overruled by Greens leader Richard Di Natale as the minor party squared off against Labor in the Batman by-election.
Senator Di Natale warned the Greens had concerns with the tax plans and vowed the Greens would use their Senate numbers to fix the policy under a future Labor government.
He urged conservative voters and those considering staying home in last Saturday's by-election to "say what you think about Bill Shorten's attack on so many people in this community."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Labor senator Anthony Chisholm said the Greens sold out their principles to rescue what had become a "flailing campaign" in the Victorian seat.
"This mob aren't a patch on the Bob Brown Greens from a decade ago, they are selling out their principles, and they were sold short in Batman," he said.
Labor brought on a vote on Wednesday to demand all parties committed to progressive taxation support its proposed reforms to cash refunds for dividend imputation credits.
Senator Di Natale changed his tune to side with Labor on the vote, but claimed the opposition had adopted a Greens idea to reform franking credits.
"Our position is clear - we support reforms to franking credits - that's why we support this motion," he said.
"But we will do our job in the Senate, and when you finally decide what your policy is, we'll make sure that if it hurts struggling pensioners, we'll protect them."
The vote was ultimately defeated 32 to 29.
