Just last months, the Democrats won back control of the US Congress in mid-term elections – in the case of the Senate, by the narrowest of margins. The new members and senators take their seats on 3 January. But yesterday, there was a fear that the Democrat majority could already be in jeopardy.

Democrat senator Tim Johnson of South Dakota is in intensive care after suffering what The New York Times describes as “an unusual form of a stroke”. He was said to be “recovering without complication”, but it is too early to offer any long-term prognosis.

If Johnson were to die or resign, the governor of South Dakota, Republican Michael Rounds, would appoint a replacement to serve until November 2008. If he appointed a Republican, which he is legally entitled to do, that would produce a 50-50 tie in the Senate, and the casting vote of vice-president Dick Cheney would give the Republicans a majority.

This is all déjà vu for a certain generation of Australians. In our Senate, casual vacancies are filled by appointment of the relevant state parliament. Over decades, a convention developed that they should always choose a nominee of the same political party as the dead or departing senator. But that convention broke down during the time of the Whitlam government in the early 1970s.

The most significant case was the death of Queensland Labor senator Bert Milliner in 1975. The Queensland parliament refused to appoint Labor’s nominee, the later-notorious Mal Colston, to replace him, and that change in the Senate’s numbers was crucial to the Coalition’s ability to block supply later that year, ultimately leading to Whitlam’s downfall.

America has much the same issues. Casual vacancies in the House of Representatives are filled, as in Australia, by holding by-elections (there called “special elections”). For the Senate, the 17th amendment also provides that “the executive authority of such State shall issue writs of election to fill such vacancies”, but adds a proviso that state legislatures “may empower” the governor “to make temporary appointments until the people fill the vacancies by election as the legislature may direct.”

South Dakota, like most states, has done so; the governor’s nominee holds office until the next regular election, which will be in 2008.

Since US senators are elected one at a time, there is (unlike Australia) no issue about proportional representation, so no reason not to hold by-elections – except that states dislike the cost of an extra state-wide election.

The Australian same-party convention was written into our constitution in 1977. If Johnson is unable to return to his duties, it may open debate on taking remedial measures in America as well.