Since the conventions, a key question in the US presidential race has been whether Donald Trump could hold together a substantial Republican voting bloc - and what that bloc might look like.

When his poll numbers tanked shortly after the Democratic convention (and Trump's fight with the Khan family), it was largely a function of Republicans bailing.

In more recent national polls, which have shown the margin between Trump and Hillary Clinton all over the map, we could look at how much support Trump got from Republicans as key factor driving the results.

The Washington Post's tracking poll, conducted with ABC News, has shown a much closer national race over the past few days, thanks in large part to Republican enthusiasm.

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"Greater Republican unity has buoyed Trump's rising support," our pollsters wrote of today's poll, which showed Trump down a single point. On October 28, Trump trailed by six points. Since Republicans vote more consistently than Democrats (since consistent voting correlates to age and income), more support from Republicans can quickly mean a more Trump-friendly likely-voter pool.

But although Republicans are embracing Trump, it's not the case that this is the same old Republicans finally settling on Trump. As from the beginning, Trump's base looks quite different from Mitt Romney's.

We can compare the voting history of various groups with numbers from recent Post-ABC tracking polls to make that point obvious.

Men usually vote Republican and Trump leads with them. He's improved five points with men since that October 28 tracking poll and he's now doing two points better than Romney did in 2012. While he's still losing women, he's actually doing three points better than Romney did with the group, after an eight-point shift since the poll on the 28th. Margins of error apply here, of course, so that shift may look bigger than it actually was.

But other numbers show where that shift happened. Trump's doing five points worse with whites at this point than Romney did, thanks to the fact that he's doing worse with college-educated white voters. He's doing 18 points better with non-college-educated white men and seven points better with non-college educated white women than Romney did - but substantially worse among those with college degrees.

Since October 28, though, those numbers have ticked up a bit. He's improved three points with white women with college degrees, and seven with white men with degrees. Among those voters, the race is now basically tied - though Trump's still doing worse with the group than any Republican in decades.

College-educated white men and women tend to be more bipartisan than those without degrees, but, overall, college-educated whites have backed the Republican consistently in presidential balloting. Trump's down 12 with that group in total - but was down 17 on the 28th. (Subsets of subsets of voters have higher margins of error, so keep that in mind.) Even among white voters who are more sceptical of his candidacy, Trump's been doing better.

The white Republican vote - which could also be simply described as "the Republican vote," since most Republicans are white - isn't the only factor. Since he was down six points in our tracking poll, Trump's made other gains, including among independents.

At his best in polling, Trump's been tied with Clinton, as he is now. In the case of a tie, things like voter enthusiasm play a big role: the extent to which voters are motivated to go to the polls to cast a ballot can make a huge difference. This can be hard to track; it can depends a lot on how good a candidate's campaign efforts are on the ground, for example.

But there was room for Trump's support to grow within his own party a week ago. It did. That base continues to be more heavily made up of whites without college degrees than was Romney's four years ago, but a vote is a vote.