Sinn Féin's Michelle Gildernew has said unionist farmers will help her retain her Fermanagh and South Tyrone seat.
She said pro-Remain unionists see her as a better option than her Leave-voting unionist rival, Tom Elliott.
Ulster Unionist Mr Elliott, a unionist farmer himself, said he is still a "soft" leaver but agrees with his new party leader, Steve Aiken, that remaining in the EU would be better than the terms of the withdrawal deal.
However, he believes a no-deal exit would also be preferable to the proposed UK/EU agreement.
With the constituency having been decided by the narrowest of margins in recent history - a solitary vote in 2010 and just 53 in 2001 - incumbent MP Ms Gildernew claims the backing of pro-Union voters could be key on December 12.
"The Brexit thing is having an impact and people who maybe still wouldn't necessarily agree with me on a constitutional position, agree with what I'm saying and what Sinn Féin are saying on the Brexit issue and they will vote," she said.
She added: "Unionist farmers are looking at their future in a post-Brexit UK, and they're frightened, and they've very good right to be."
Mr Elliott, who won the seat from Ms Gildernew in 2015, only to see her regain it in 2017, said he does not believe Brexit is the issue per se, rather the length of time the process of leaving the EU has taken.
"I always felt there was an easier resolution to the whole Brexit issue - and that was by the European Union giving special status to the Republic of Ireland to trade freely with a UK which includes Northern Ireland, and vice versa, and I think that would have resolved a lot of issues because we are two islands," he said.
The SDLP's Adam Gannon, the Alliance Party's Matthew Beaumont, and independent Caroline Wheeler are also running in the seat. The DUP has stood aside to give Mr Elliott a better chance of winning.
