Tony Blair will on Thursday night become the first Labour leader in British political history to win a third successive general election victory. Of that, there is not too much doubt. There is always scope in any election for something extraordinary and unforeseen to happen. But we can be reasonably sure that, in the early hours of Thursday morning, Mr Blair will secure the historic achievement that has eluded all his Labour predecessors.

In spite of that, one big question still hangs over this result: what will Mr Blair's majority turn out to be? The assumption of most independent analysts last night was that it would come somewhere between 60 to 120. But where it falls precisely in that range is critical to the prime minister's hopes of securing a radical reforming third term.

Thursday morning's final polls in four national newspapers showing Labour with leads of between 3 and 6 points over the Tories ought to give Labour plenty of comfort about the majority. But by some technical quirk, opinion polls have consistently over-estimated the size of Labour's lead.

After seeing Thursday night's poll one cabinet minister was still fretful, assuming the majority would come at the low end of the range at about 70. This may be a neat bit of expectations management, lowering the target to make the eventual result look better. But there is no doubting the significance of Labour's new majority.

If it is at the high end of the range, then Mr Blair has all the momentum he seeks for his third term. But if the majority is closer to 60, life will be far tougher not least because some 50 core Labour rebels are expected to be returned to the Commons and will block much of the reforming drive Mr Blair plans in public services.

A majority in the middle about 90 to 100 is the most likely result in the view of many pollsters. But even here, Labour's spinners would have to work hard to put a gloss on the result.

First, a majority of 100 would still see Labour losing about 30 seats the party has not seen losses on this scale since 1983. That will take some of the pleasure out of a third term victory for many in the party. Second, Labour may have to work a little to remind people that a Commons majority around the 100 mark is high by the standards of postwar politics. In 1997 and 2001, Labour enjoyed massive historic majorities of 179 and 165. But these were exceptional. Of the 16 postwar general elections, only six have seen the winning party get a majority of more than 100.

Labour strategists were already beginning to make such arguments yesterday. One pointed out that, back in the mid-1960s, Harold Wilson was grandly claiming that Labour was the “natural party of government”, and this on the basis of a majority of 96 secured by Labour in the 1966 election.

The most critical issue on Friday morning, however, may be the extent to which Labour's majority is underpinned by a strong share of the national vote. If Labour is to have additional legitimacy, it would want a parliamentary majority to be accompanied by a high share of the national vote cast.

A lot of attention will also be placed on the Conservative share of the vote. If the Tories end up having a share that remains in the low 30s, there is no chance of them winning the next election at the end of this decade.

But the dynamics of the first-past-the-post electoral system could work against Labour. It is not inconceivable, for example, that Labour could secure a majority of 84 on the basis of a 34 per cent share of the vote, 2 points ahead of the Conservatives on 32 per cent. A result like this, especially if based on a low overall turnout of voters, would leave Labour's opponents arguing that Mr Blair's victory was a triumph of electioneering over democracy.

Ultimately, Labour's potential momentum may depend on other factors, not least the relationship between Mr Blair and Gordon Brown, the chancellor. New Labour's success in recent days has been built on the fact that its two co-founders have firmly and visibly come together. If that co-operation can continue in the next few months, moving to common positions on many of the big policy issues such as pensions and public spending, the Blair government will start an important new chapter.

But on Friday we shall all be poring over the hard statistics, the governing majority, the turnout and the shares of the national vote.

That will tell us what Mr Blair is in a position to do in the government and for how long.