The resurrection of the biggest rivalry in territory politics, between Katy Gallagher and Zed Seselja, has sparked speculation it could endanger the Liberals' ACT Senate spot.

But past election results suggest they will face each other across Parliament's red chamber for years to come after the next federal election.

Senior Liberal Party sources said there were concerns about the damage the immensely popular Ms Gallagher could do to Senator Seselja, particularly if the problems the Coalition faced this week did not go away.

But two years ago both arguably laid claim to being the most popular politician in the Legislative Assembly.

It was the year where an underestimated Mr Seselja as Opposition Leader almost dislodged Ms Gallagher as Chief Minister.

He failed in his bid after she struck a balance-of-power deal with Greens MLA Shane Rattenbury.

The high-profile Ms Gallagher as Chief Minister drew 23,996 first preference votes in Molonglo in 2012, or more than double her quota, meaning she pulled another Labor candidate into the Assembly with her.

In comparison Mr Seselja attracted 18,566 votes in Brindabella in a smaller electorate.

Mr Seselja got 29.4 per cent of the Brindabella vote while Ms Gallagher got 24.9 per cent of the Molonglo vote.

Making it unlikely Ms Gallagher will directly threaten Senator Seselja's spot is the fact the battle in the ACT is usually focused on the stoush for the remaining No. 2 Senate spot that the Liberals took comfortably last year.

The stoush between Mr Seselja and the Greens' Simon Sheikh ended with the Liberals almost doubling the grouped votes of the Greens in 2013.

Labor's Kate Lundy held the No. 1 ACT spot by a strong margin.

After Ms Gallagher's announcement on Friday Senator Seselja said: " "It's an old rivalry rekindled - I've always enjoyed the rivalry.

"I did jokingly say she'd miss me when I left the Assembly.

"Obviously I'd wish her well - it's not an easy transition."

Even though Lake Burley Griffin has separated Senator Seselja and Ms Gallagher in the past 18 months the combative relationship has not died between the Catholic father of five and the former disability worker and mother of three.

Senator Seselja said Ms Gallagher would be joining an obstructionist federal Labor party and that she was leaving her ACT Labor colleagues lumped with a financially unsustainable and politically sensitive light rail project which she committed them to.

"Katy Gallagher won't be there to fight the battle [on light rail]," Senator Seselja said.

"I don't think the community wants light rail."

Ms Gallagher showed no mercy on Mr Seselja when stepped down as Opposition Leader before running for the Senate. She said he should leave the Assembly immediately.

She leaves the Assembly this month.