A phone survey conducted by London polling agency Survation put Tory candidate Jill Mortimer in the lead with 49% and Labour’s Dr Paul Williams in second on 42%.
Voters will go to the polls on Thursday, May 6, after Labour MP Mike Hill’s surprise resignation on March 16.
Hartlepool has been a Labour seat since 1964. But the town has come under increasing competition from other parties since Hartlepool voted by 69% to leave the European Union in the 2016 referendum.
In the 2019 General Election Mr Hill held on to the seat he first won in 2017, but saw his majority more than halved to 3,595.
The by-election survey was conducted on behalf of the Communication Workers Union.
Survation stated: “At this stage in the contest, it would appear a significant proportion of 2019 Brexit Party voters have moved to the Conservatives, have yet to make up their minds or will choose not to vote.
“How this develops will have a significant effect on the final outcome."
Prime Minister Boris Johnson also had a higher favourable rating in the poll than Labour leader Sir Keir Starmer at 49% compared to 24%.
Survation added: “A potential explanatory factor for the Conservative lead in Hartlepool, alongside a ‘vaccine bounce’, is the relative differences in how voters in Hartlepool view the leaders of the two main parties.”
The significance of the by-election to both parties was shown with both the PM and Keir Starmer visiting Hartlepool for the first time last week.
Boris Johnson toured specialist medical diagnostics manufacturer Hart Biologicals on the Queens Meadow Business Park followed by an impromptu walkabout on the Fens estate.
The phone poll put Thelma Walker (Northern Independence Party) in third place on 2%, The Green Party, Andy Hagon (Liberal Democrats) and John Prescott (Reform UK) each on 1%.
Hilton Dawson (North East Party) is currently on less than 1%, and other candidates account for 4%.