The battle for the plum post of ACT Liberal senator is now a contest between the voice of experience and the wind of generational change.

It sparked a surprisingly tough warning from Gary Humphries that Canberra was going to be slammed by an Abbott-led government.

So Liberal Party preselectors need to keep him in the job, he says, because he has the experience to be Canberra's ''champion''.

Humphries is trading on his record as the only Canberra Liberal to serve on the Coalition frontbench, along with being willing and prepared to take on the federal leadership. And late on Monday Tony Abbott endorsed his frontbencher.

Publicly Humphries welcomes this exercise in democracy. He must have seen it coming - everyone else did.

Zed Seselja turns 36 next month and is pitching himself with the usually reliable theme of ''renewal''.

He is aligned with Abbott's conservatism whereas Humphries is a moderate who crossed the floor on a matter of conscience.

The Liberals dispute the notion that Seselja's preselection would be a gift to the Greens' candidate, Simon Sheikh. But a Greens strategist says the party believes Seselja will be an easier target to attack by linking him to Tony Abbott's conservative policies than Humphries.

However, it is almost impossible for Sheikh to win, given ACT Senate history, and that was before the Greens were almost wiped out in the recent ACT election.

Seselja's unsurprising backflip means he has to contend with criticism he is ''abandoning'' the residents of Tuggeranong. But did anyone really expect him to stick around after doing an excellent job of pulling the Liberal Party into a cohesive unit but still missing the prize by a whisker?

His team is telling anyone who will listen that he pulled more votes in the valley than the Greens and Labor combined.

How to utilise this popularity?

Grassroots members of the Liberal Party may reflect that if the ACT Opposition Leader really wanted to help bring about an Abbott-led victory, he might trade on his following on the south side to challenge Labor's Gai Brodtmann for the seat of Canberra.

But he knows that move is risky, given a likely backlash against Abbott's plans for the public service and, even if he won, he would not hold the seat beyond one term.

Seselja saw Brendan Smyth feted as a champion for taking a Labor seat only to be booted out after one term. So, Seselja is taking the less risky option.

It is significant that he wouldn't mount the challenge against the incumbent if he didn't have the numbers.

A senior party member all but confirmed this.

If Seselja wins preselection in three weeks, a question of principle must be addressed.

Technically he could stay on the backbench in the Assembly until the writs for the federal election are issued in August but to stay even one day after gaining Liberal Party preselection for the Senate would be questionable.

As soon as he picked up the telephone, he could be accused of using ACT taxpayers' money to campaign for the Senate seat, whether he was doing so or not.

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