Barack Obama and John McCain swept triumphantly up the Potomac yesterday, scoring big wins in the latest three states to vote in the US presidential primary election.

For Mr McCain, victories in the mid-Atlantic contests of Virginia, Maryland and the District of Columbia marked merely another set of mileposts on his now inevitable path to the Republican nomination.

The much more important question is, did Mr Obama’s even bigger triumphs set him at last and decisively on the parallel path to the Democratic nod? The Obama sweep was impressive in three ways.

Scale

First it was large. He won Virginia by almost two-to one over Hillary Clinton. He won Maryland by about three-to-two. And (least surprising of all) he won the District of Columbia by almost three to one. In the Democratic race, where delegates to the nominating convention are awarded in almost direct proportion to votes received in each state, this is a big deal. And for the first time since Iowa, his haul of delegates put him ahead of Mrs Clinton (albeit narrowly) in the race for delegates to the party’s nominating convention.

Second, it promised, perhaps, for the first time, to give Mr Obama the elusive momentum both candidates have sought in the Democratic race. With his victories in five contests at the weekend (Washington, Louisiana, Nebraska, Maine and the Virgin Islands) his Potomac wins extend his streak to eight states since Super Tuesday a week ago. The next states to vote – next Tuesday are Hawaii – Mr Obama’s home state, and Wisconsin, a famously progressive state that might well also be on the way into Mr Obama’s pile. That would make it ten straight wins to Mrs Clinton’s zero.

So far no-one in this campaign has established momentum. Every time someone looks like establishing a decisive lead, they get derailed. Mr Obama seemed to have momentum after the very first contests in Iowa six weeks ago, but lost it in New Hampshire five days later. Then after his big win in South Carolina in late January he came up against Mrs Clinton’s coast-to-coast stand on Super Tuesday that at least held him to a tie.

Might it be different this time?

Hard to say. But it is now clear that Mrs Clinton simply must win at least one and probably both of the big contests that follow on March 4 – in Ohio and Texas. The decision in the Democratic race is probably going to come down to the decisions of the so-called superdelegates, the party’s elite at the federal and state level, who represent 20 per cent of the total delegate strength at the convention. Though a majority of those had been thought to be inclining towards Mrs Clinton, if they see Mr Obama surging across the country, they are very likely to jump into his camp and deliver the death blow to Mrs Clinton’s campaign. She simply has to stop him from achieving that on March 4.

Breaking the logjam

But the third and biggest reason to think Mr Obama’s Potomac wins may have been significant shows up in a detailed analysis of the voting. There were signs on Tuesday that he might just have started to break the stalemate in the Democratic race.

Until yesterday, the two Democratic candidates had been locked in an almost perfectly balanced struggle between their two demographic coalitions. Mr Obama had the support of blacks, young voters, independents and new voters and the relatively affluent – the latte liberals, the sort of college educated professionals who spend a lot of money on expensive coffees in Starbucks.

Mrs Clinton has women, Hispanics, older voters and the traditional Democratic base – the relatively less well-educated and poorer blue collar workers – the Dunkin Donut Democrats.

But in Virginia and Maryland, Mr Obama not only held on to his demographic groups – increasing his leads among blacks and young and independent voters – and ate substantially into Mrs Clinton’s core vote.

He beat her in fact among almost all categories of Democratic voters – the poor as well as the affluent, the high school graduates as well as the postgraduates. He won a majority of Latino voters and most impressively, he lost only narrowly among all white voters, keeping his big lead among white men and even pushing Mrs Clinton fairly close among white women.

It’s possible that there is something very particular about these mid-Atlantic states that voted Tuesday. The black vote was large in Maryland and DC; in Virginia he probably benefited from the votes of large numbers of independent and Republican voters who crossed over to vote in the Democratic primary. Almost a third of the voters in the Democratic contest there were independents or Republicans.

But even allowing for the unusual nature of these states, there were more than just faint signs that the tectonic plates of the Democratic primary are shifting. Mrs Clinton spent Tuesday night – and will be spending quite a lot of the next three weeks – in Texas. How fitting for this odds-defying political survivor to be preparing to make her critical stand in the state made famous by the Alamo.

Times Recommends

• Baghdad fury at claim US spied on PM
• British envoy criticised over Swaziland party
• How perfect is Sarah Palin’s First Dude?