Trump was a response to the bipartisan political consensus, the main pillars of which were (and still are): immigration is great, America needs to be involved in the middle east and elsewhere, free trade is awesome. This had been the consensus for decades at that point, and anyone who disagreed was a rube.
The thing is, i don‘t think the Republican base was ever really into any of those things. They always wanted to restrict immigration, were never the biggest fans of free trade, and didn‘t really care about the middle east until 9/11. Trump was just the first to say it openly. The Republican leadership had been completely disconnected from their base for a long time, probably since after Reagan. The kind of Neocons that have dominated the party since the end of the cold war have always been progressives hiding behind a veneer of militarism and America-fuck-yeah patriotism, with the exception of some social conservatives like Santorum or Huckabee.
Or, as i‘ve heard it being said on a podcast some time ago: Modern conservatism is merely progressivism in slow motion. I don‘t know if Trump could have won in 2008 nationally, but i could definitely see him winning the republican base over.
The more interesting question is, when would be the latest that Trump could run as a Democrat, but with more or less the same platform as 2016? The 90s, maybe? I think being hawkish on immigration and being against stuff like NAFTA, while also being fairly liberal socially, could be a successful platform for a Democrat as late as 1992. Trump would just fit perfectly into the culture of the time. Though he likely wouldn‘t be very successful as a Republican in that era, as he might be seen as too socially liberal for the religious right, which was much stronger back then.