Nick Xenophon is quitting the senate to return to South Australian politics, in a move with big ramifications for both the federal and South Australian arenas.

Xenophon has been in the Senate for nine years, for much of that time as a key swing vote on legislation — as well as period of, by his own happy admission, being a “feather duster” without legislative influence. But today he announced he would be running in the South Australian state election next March for a lower house seat, the marginal Liberal-held seat of Hartley. He’ll also be leading the SA-BEST team in, according to Xenophon, more than a dozen seats. Hartley is mostly within the federal electorate of Sturt, Christopher Pyne’s seat.

Xenophon’s third-party assault on South Australia poses real dangers for both the Labor and Liberal parties. Xenophon secured three Senate spots in the 2016 double dissolution election but also managed the impressive feat of seizing a House of Representatives seat from the Liberals, when Rebekha Sharkie won Mayo.

NXT polled nearly 22% in the Senate and 21% in the lower house in South Australia — a surprisingly consistent performance that suggests he can inflict major damage on both the government and opposition. Psephologists will now be rapidly recalculating their forecasts for the coming election to determine how many lower house seats Xenophon can potentially pick up, and his chances of securing the balance of power in the lower house and thus deciding who will govern.

In his time in the Senate, Xenophon has combined a brilliant instinct for publicity with a stubborn centrism and deep commitment to the anti-gambling cause that first propelled him into South Australian politics, and to protectionism, particularly on manufacturing and foreign investment. It was the electoral threat posed by Xenophon that helped push the Coalition into embracing a local build approach to Australia’s next generation of submarines and other vessels.

Most recently, Xenophon helped negotiate the overturning of most of Australia’s remaining media ownership restrictions in exchange for federal government investment in regional and small media. He was also an outspoken voice on civil liberties, and with the Greens has been a rare source of consistent scrutiny of intelligence agencies and proposals to give them greater powers.

Xenophon’s announcement comes ahead of next months’ determination by the High Court of whether he has been eligible for election to Senate due to holding British citizenship via his Cypriot father; if ruled ineligible, it would have been straightforward for him to engineer a return to the Senate. Now, however, it appears that the High Court will determine whether NXT’s Tim Storer is deemed elected in Xenophon’s place, or whether the party can appoint a replacement if Xenophon’s candidacy is deemed to have been valid.

His departure from the Senate leaves Stirling Griff, elected last year, as most the senior party figure and likely leader who will now guide the party in its negotiations with other parties on the fate of legislation in the Senate.