Leading article: Decision day for a nation emerging from Bush's shadow

There has never been a "Super Tuesday" like it. Voters in 19 US states will today decide the allocation of their delegates to the party conventions this summer. There will be Democrat-only votes in three more states and Republican-only contests in another two. In all, there will be voting in 24 states, with the biggest prizes being California and New York – states big enough and rich enough to be countries in their own right.

What makes today so special, though, is not just the record number of states voting, but the level of engagement of the voters and the likely record turn-outs. With both party contests still open, despite the exceptionally early start to the electoral season, there are compelling reasons for everyone who has a vote to use it.

Connections and dollars may dictate who gets a chance to run for President at all, but so – at this crucial stage – do rank-and-file party supporters. It is they who will decide whose name eventually appears on the ballot paper. The primary campaigns are by way of being the ultimate interview process for the party's nominee. Voters will choose not necessarily the individual they prefer as a person – although some will do this – but the one they think would make the strongest candidate for President. On this make-or-break day, voters might make a different choice from the one they would have made earlier in the primary season.

For the Republicans, "Super Tuesday" looks likely to be decisive. As the campaign has progressed, John McCain has been able to recover some of the straight-talking glitz that won over such a broad swathe of primary voters in 2000. Whether it reflects regret over the preference given to George Bush in 2000, hesitation about the national electoral prospects of a Mormon, or the simple fact that Mitt Romney's campaign has so far failed to catch alight, today is looking increasingly like the former Massachusetts governor's last stand.

If this turns out to be so, however, Mr Romney can reasonably share the blame for his failure with George Bush. Mr McCain's growing advantage in recent weeks has been that he has come across as the furthest – in character and policies – from the outgoing President. He may have supported the "surge" in Iraq, but he publicly disagreed with Mr Bush over the "lighter, nimbler" force dispatched – on Donald Rumsfeld's advice – to effect regime-change. On the environment and many – but by no means all – areas of social policy, he is someone who could plausibly take on either of the duellists fighting for the Democratic nomination.

One early conclusion from the primaries is that whoever wins the White House – Republican or Democrat – will not be a proxy third-term George Bush. American voters are clearly impatient to see the back of him. That probably bodes well for the future direction of US foreign policy and may help to explain the excitement with which this campaign is being followed abroad.

The other reason, of course, is the vivid, all-American cast of characters. The promise of the first woman or first black American to head a US presidential ticket is epoch-making in itself. But the frenetic pace and switchback nature of the Democratic contest has shown off the US election process at its thrilling best.

This was the earliest start to a presidential campaign ever, yet it has many – in the US and elsewhere – hoping that it will not end when the counts in the 24 primary states are over in the early hours of tomorrow morning. With the polls for the Democratic primaries narrowing all the time, we dare to hope that the race will go on – perhaps even to the party convention in Denver at the end of August.