Democratic pollster supremo Stan Greenberg was in bullish mood at an IPPR roundtable last week discussing the effect of the upcoming US election on the progressive project.

Greenberg frequently predicted that the forthcoming vote would be an ‘earthquake election’, and compared the potential outcome to the rout of the Democrats in 1984 – except this time the Republicans will suffer.

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In a wide-ranging talk, Greenberg examined not only the electoral maths that is opening up previously unwinnable states as the gap between Clinton and Trump widens, but also the effect that a Clinton win would have on the future of US politics. He argued that Clinton – in part because she was pushed to the left by Saunders in the primaries, and in part due to the evolution of her own thinking – is running on the most progressive platform since Lyndon Johnson in the 1960s.

This will mean wide-ranging change under a Clinton White House (especially if the Trump effect means that congress is also controlled by the Democrats). Whoever controls congress, the makeup of the supreme court will change. This, in turn, may mean the overturning of the Citizens United ruling, which has long been a block on serious campaign finance reform.

Equally, while Clinton may not be as progressive on Wall Street reform as some would like, she has a strong agenda on corporate governance that could have long-lasting consequences. Reigning in excessive executive pay, increasing profit-sharing and the minimum wage, and ensuring that the wealthy and corporations pay a fair share of tax are all a significant part of Clinton’s platform. If she can pass this platform and embed it, then 8 November 2016 will be looked back upon not just as an election, but a significant turning of the tide of US politics against conservatism.

This is possible not just because of the implosion of the Republicans – though this has helped. The demographics of the US are shifting rapidly, particularly towards the cities where a vast majority of voters live, and which are made up largely of college-educated voters and minorities. Their suburbs – once the preserve of the ‘white flight’ Republicans – are becoming increasingly diverse and Democrat-leaning, too. This bodes well for progressive Democrats, who have led among these two groups for some time.

The Republicans, meanwhile, will find themselves in the midst of an intensifying internal civil war. Even with a catastrophic loss, there is little doubt that Trump will remain a key player; it is also likely that most of those who lose their seats in congress will be more moderate Republicans. Those in the party horrified by its current populist turn may find themselves increasingly isolated, and further from the levers of power than ever. As demographics shift against them, and the party risks turning ever further inwards, the GOP may face a long, hard fight back to electability.

However, Democrats should not be complacent or triumphalist. The media will inevitably paint this not as a Clinton win but as a Trump loss. Presidents inevitably disappoint, and Clinton’s favourability ratings are not particularly high to begin with.

There are a lot of Democrats up for re-election in two years’ time, and that may be focus enough for the Republicans to get their act together – at least temporarily. Clinton must act fast to advance the long-term progressive agenda that this moment allows. Once that is established and normalised then it can be built on – by Clinton or whoever comes next.