The grip of the big-C Conservatives on the federal Coalition may be about to tighten, with moderate ACT Liberal Senator Gary Humphries likely to be rolled in a preselection by a man seen by some as a youthful Tony Abbott wannabe.

Local ACT Liberal leader Zed Seselja yesterday ended months of claiming he wouldn’t challenge Humphries for the party’s number one ACT Senate spot by standing down to contest Senate preselection on February 23. The ACT elects two senators for three-year terms; the first seat will likely go to Labor’s Kate Lundy while the second will go to the Liberals or the Greens.

ACT Liberal circles are abuzz at the Seselja raid, and almost all party insiders think he’ll have the numbers to roll Humphries. That’s likely to mean one less bum on a seat for Liberal moderates in the Senate, and one more conservative one.

Seselja is a 35-year-old Right-leaning Catholic who pulled in a surprisingly strong result in last year’s ACT election and has a solid following in the territory’s party branches, which are Right-leaning and dominated by a religious power clique. But Humphries won’t go down without a fight, and Seselja’s raid on a sitting senator has annoyed some — particularly moderates — within the federal party. Seselja and Humphries met in secret to discuss the challenge on Sunday night.

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“Gary’s really disappointed,” an insider told Crikey. “It’s going to get nasty. Gary’s a fighter, he’s a career politician and I can assure you he’s not going to walk away. He’s in the fight of his political life.”

Humphries has sat in the Senate since 2003. He famously became the first Liberal senator to cross the floor in the life of the Howard government, when he voted with the then-opposition in 2006 to defend the right of the ACT Parliament to enact same-s-x union laws. Humphries has also been relatively outspoken on the impact on Canberra of the Liberals’ proposed sweeping cuts to the federal public service. Seselja is seen as being less likely to depart from the party line than Humphries; he’s an admirer of Abbott.

Abbott yesterday supported Humphries in the preselection stoush, telling Fairfax: “As a former chief minister, Gary has been an outstanding advocate for the ACT and contributor to my shadow ministry. He has my support.”

However, a federal Liberal insider told Crikey Abbott had to publicly side with Humphries because of the drama over Julia Gillard’s “captain’s pick” of Nova Peris against sitting NT Labor Senator Trish Crossin. The source speculated Abbott was unlikely to seriously intervene to help Humphries.

Humphries has long been vulnerable to a preselection challenge because his moderate politics don’t resonate with the Right-leaning Canberra Liberals. “People think Gary’s doing a good job, but no one is surprised [about Seselja’s challenge],” the Liberal source said. “I suspect Zed will win … I think people will be disappointed to see Gary go.”

Another federal party source said Humphries was “totally getting rolled … doesn’t have a chance”. Some think Seselja should have run for the lower house seat of Canberra, where he would stand a chance of unseating Labor’s Gai Brodtmann; a net gain of a seat to the Coalition.

Seselja defended his move on a sitting senator, telling Crikey it’s “part of the democratic process. No one is immune to challenge.”

He says the Liberals haven’t been maximising their ACT vote in the Senate: “We do need to strengthen our vote in the Senate.” He concedes the Liberals are unlikely to lose the seat to the Greens either way, but says he would be a good asset in the Senate, although he had considered a lower house run.

Seselja rated his chances at preselection as “quite strong … the party has fared well under my leadership. I’ve obviously been chatting with members and gauging whether or not they support me.”

Canberra Liberals will individually cast their votes for preselection; it’s not done on a branch-by-branch basis. It’s understood just two candidates have nominated, which would mean no preferential preselection.

Whoever wins Liberal preselection will tussle it out with the Greens’ Simon Sheikh for the ACT’s second Senate seat at the September 14 election, with the winner to take their seat immediately (unlike the state senators). The Greens have been talking up their chances at winning the seat, as they do each election, so far without success. ABC election guru Antony Green yesterday said it was possible but unlikely the Greens would win the seat.

In 2010, Humphries polled bang on a quota (the Libs got 33.4% of the primary vote), while the Greens got 22.9%. For the Greens to win the second Senate spot, they’d need the Liberal vote to drop below a quota, so Sheikh could get over the line with Labor preferences (Labor scored 1.2 quotas last time).

Green points out this is perhaps not likely given the Coalition is polling well across the country. However, a complicating factor is that Abbott’s planned cuts to the public service may cost votes in the territory.

Seselja will stay in the ACT Legislative Assembly for the time being, although he has resigned as local Liberal leader — the party will choose a new leader on Monday.

Another issue to watch is what happens to ex-Seselja staffers Tio Faulkner and Stephen Doyle, who are both ACT powerbrokers and could crop up at the federal level in the future. Both resigned from Seselja’s office late last year. Faulkner remains the ACT party president.