According to pollster Mario Canseco, the Yes side is still leading with 46 per cent support compared with 42 for the No side.
"While the Yes side is still ahead, the proportion of definite supporters remains low," said Canseco on Tuesday.
"Drivers are now more likely to say they will vote No, and the level of support from transit riders, cyclists and walkers has dropped noticeably in a month."
Canseco points to a lack of information on what a Yes vote would mean for communities as a factor in softening support.
"With only a few weeks left before the ballots are mailed out, most residents are unacquainted with what a victory for the Yes side will mean to their community and their commute," he said.
Other concerns for those who would vote No included a lack of confidence in TransLink's performance and concerns about how the funds would be spent.
The poll was conducted by Insights West using 643 adult residents from Metro Vancouver chosen from their online panel members between Jan. 9 and 12.
Vote set for spring
The plebiscite will be conducted by Elections B.C. by mail-in ballot, which will be sent out March 16. Votes must be in by May 29.
Voters will be asked: "Do you support a new 0.5% Metro Vancouver Congestion Improvement Tax, to be dedicated to the Mayors’ Transportation and Transit Plan? Yes or No."
A simple majority of 50 per cent plus one vote will be considered support for the question.
The new 0.5 per cent tax would apply to the majority of goods and services sold or delivered in the Metro Vancouver region and bring in an estimated $250 million in annual revenue.
The region's mayors have said it would be used to fund part of a $8 billion 10-year transit plan, including light rail in Surrey, rapid transit on Broadway, and increased service on SkyTrain, Canada Line, SeaBus, and West Coast Express.