Gary Humphries is slightly ahead of Zed Seselja in a survey of Canberra voters commissioned by ACT property developers.
The two candidates will face Liberal Party preselectors on Saturday for the right to contest the federal election as the party's lead Senate candidate.
The phone poll of 800 people was conducted by Taverner Research last week and released to Fairfax Media on Wednesday evening.
While the developers would not be named, they made it clear they favoured having an ACT senator in the front ranks of a Coalition government to protect Canberra and their business interests.
They envisage a big impact on ACT property values if Tony Abbott wins the election and proceeds to cut the bureaucracy and programs.
''The survey was commissioned by a small group of Canberra businessmen who have a deep interest in the economic welfare of the ACT and who are concerned about what might happen in the event of a change of federal government,'' one of the proponents said.
''These businessmen share the view that Senator Humphries, if he is going to be in the ministry, is going to be in a much stronger position to protect the business interests of the ACT than will Mr Seselja who is really going to be a very small fish in a large pond and who doesn't necessarily at this stage have the support of his future fellow colleagues.''
The developer, who said he was not a Liberal Party member, believed Mr Seselja was the frontrunner.
Market researcher Philip Mitchell-Taverner said the survey respondents were asked about their intentions for the Senate vote if the top Liberal candidate was Senator Humphries or Mr Seselja.
''Gary Humphries is preferred to Zed Seselja overall as the Liberal Senate candidate, 46 per cent versus 33 per cent,'' he said.
''Overall, the survey shows that the Liberals should gain a stronger share of the Senate vote with Gary Humphries as their candidate, 38 per cent, than with Zed Seselja, 36 per cent, especially among Liberal voters, 93 per cent against 88 per cent.
''Further, Gary Humphries would win more first or second-preference votes, 43 per cent, than Zed Seselja, 39.''
Asked about the incumbent's performance, 47 per cent of respondents said Senator Humphries was doing a good or excellent job and 9 per cent believed he was doing a poor or very poor job.
Among Liberal voters, 59 per cent rated his performance as excellent or good, and 2 per cent thought he was doing a poor or very poor job.
Mr Mitchell-Taverner said the results also indicated a swing to the Liberals in the ACT this year, in both the House of Representatives and the Senate, of 3 to 5 per cent.
Senator Humphries was forced to defend himself about internal party criticism of his performance as long as three years ago when he was approached by a delegation of party members in 2009 that included Mr Seselja.
Senator Humphries told Fairfax Media he believed he had been ambushed with the details of the delegation being leaked to the media close to the ballot.
It comes just days after Mr Abbott warned against ''ambushes'' in preselection battles and threw his strong support behind Senator Humphries.
Senator Humphries said that after the delegation came to see him, he was promoted to the Coalition front bench as an opposition parliamentary secretary.
"The arguments about me not being sufficiently liberal - an extraordinary claim for someone who has represented the party for 20 years - was obviously debunked by the fact that the federal Liberal leader put me on his frontbench,'' he said.