Notice that Donald Trump has rarely been over 45 percent. Notice that Hillary Clinton has rarely been under 40 percent.

This is Trump's main problem. He has consistently trailed Hillary Clinton in the polling average because he has consistently trailed Hillary Clinton in the polls. Only once has Trump topped 50 percent in a poll. Only eight times has he been over 45. Clinton's only been under 40 percent six times. That overlap between them -- his ceiling, her floor -- isn't very big. The polls have often not been very close.

To get a sense for how important this is, we can look at past polling. Here's the distribution of election-year poll results (as a percentage of all polls tracked by RealClearPolitics) for the contests in 2004, 2008, 2012 and so far this year. The pattern is obvious.

The candidate with the higher average in the polls over the course of the year is the candidate who has ended up winning. To which the proper response is, "Yeah, no kidding."

But consider the numbers.

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Average poll results in 2004: Bush, 46.3; Kerry, 45.2

In 2008: Obama, 47.9; McCain, 43.5

In 2012: Obama, 47.5; Romney, 45

In 2016: Clinton, 45.6; Trump 40.7

Clinton's average is lower than any eventual winner by at least 0.7 percentage points -- but Trump's is 2.8 points lower.

This is a little unfair, since the eventual winners had three more months of polls to consider, including polls at the end of the campaign where they mostly led and polls later in the cycle when fewer voters were undecided.

So let's zoom in a bit on those figures and add only polls taken up until a week after the conventions ended.

The post-convention numbers look like this:

Average poll results until just after the conventions in 2004: Bush, 45.1; Kerry, 45

In 2008: Obama, 46.4; McCain, 43.5

In 2012: Obama, 47.3; Romney, 44.4

In 2016: Clinton, 45.6; Trump 40.7

Now Clinton is outperforming George W. Bush in 2004 -- but Trump still lags far behind.

This is Trump's recurring problem, one that's been exacerbated in polls this week that show his support from Republicans flagging: He need to increase his base of support. After his convention he did just that. After the Democratic convention, that increase evaporated.