The question was on the forefront of MPs’ minds as a rather self-satisfied looking Douglas Carswell took the oath again as the first elected Ukip MP after his thumping win in the Clacton by-election last week after defecting from the Tories.
His two sponsors – Tory Father of the House Peter Tapsell and the rather independent-minded Richmond Tory MP Zac Goldsmith – are unlikely to follow him, but the expectation now is that others will.
So far, two Tories have jumped to Nigel Farage’s anti-EU party, with Mark Reckless now favourite to win back his Rochester and Strood seat on the back of Mr Carswell’s victory.
But Ukip is suggesting that among the MPs considering a switch to them are current Labour members, particularly after Ukip narrowly failed to pull off a stunning win in a safe Labour seat, Heywood and Middleton.
The message the Clacton and Heywood results gave was summed up by Ukip’s deputy leader Paul Nuttall, who proclaimed that no seat with a 20,000 majority was now safe.
MPs can be principled individuals, but far more of them are sensitive to holding on to their seats and if they have a Eurosceptic bent – as many on the Tory and Labour benches do – then Ukip is an attractive option.
The Ukip surge is not dissimilar to what helped the Yes campaign in Scotland reach 45 per cent. There is an “anti-politics” movement which is now completely unnerving the main party leaderships.
And while Labour is watching its back in England because of Ukip, it is also now fighting a defensive core-seat strategy in Scotland because of the threat of the SNP – potentially great news for Lib Dems like Jo Swinson.
As a result, Ukip and the SNP are in some ways dictating policy. Because of Ukip, both Labour and the Tories are toughening up on immigration, and the chances of an in/out EU referendum in the next parliament are looking much stronger. Meanwhile, the SNP threat is radically changing the UK’s constitutional set-up.
But it is also fuelling talk of leadership challenges. In the past week, suggestions have been made about sacked environment secretary Owen Paterson trying to replace David Cameron and former home secretary Alan Johnson challenging Ed Miliband.
The dying days of this parliament is seeing a huge shake-up of the status quo and nobody knows how it will end up.