(CNN) President Donald Trump's approval rating among voters has been stuck at or below 45% for pretty much his entire presidency. And, at this point in the campaign , a record percentage of voters say they'll vote against him .

So how does Trump win a second term? The answer may be that the Democrats nominate a very liberal candidate, something Democratic voters seem willing to do.

Further, Harris, Sanders and Warren are considerably further left of the 43 House Democrats who won seats from Republicans in 2018 -- the candidates who gave the Democrats their House majority.

The early evidence suggests that being seen as very left could make the difference. A recent ABC News/Washington Post poll found that Trump would beat a candidate perceived as a socialist by a 49% to 43% margin. That same poll gave Biden a 10-point advantage over Trump, while all the other top Democrats were either tied with (Warren), up by a point (Sanders) or up by 2 (Harris) over Trump. While some of this is likely because of name recognition -- though, as I've noted before, Warren and Sanders are well known -- the more liberal candidates have consistently done worse than Biden has against Trump.

Biden's ahead of Trump 49% to 41% in an average of more than 20 national polls taken since January. In these same polls, this 8-point lead for the Democrat slims to 5 points against Sanders (47% to 42%) and to 1 point against Warren (44% to 43%). Biden's better standing in the general election polls is driven by him being far better liked by non-Democratic voters than Sanders or Warren.

These polls look a lot like what you'd expect given research on the topic. A hallmark study by Stanford University's Andrew Hall and Daniel Thompson looking at congressional races from 2006 to 2014 demonstrated that more moderate candidates performed more strongly. This is because extreme candidates on one side tend to raise the opposition's turnout. (This runs counter to the argument that base candidates raise turnout to help their side win. Instead, it may be the opposite.) My own simple examination of the 2006-2018 House returns indicates that incumbents with more moderate voting records continued to have an advantage, once we control for a district's voting history and money spent by the campaigns.

The sample size for presidential elections isn't anywhere as large, but ideology probably also matters there. FiveThirtyEight's Nate Silver has shown that since the 1940s, more moderate presidential candidates do better on average

Ideology may have been part of the reason Trump won in 2016. He was seen as the least conservative Republican presidential nominee since at least Gerald Ford in 1976. Trump's ability to escape the very conservative label likely had to do with how he mixed in more moderate rhetoric on everything from infrastructure to the social safety net and even gay rights.

Now, to be clear, having a liberal voting record and being seen as liberal by the voters don't necessarily go hand-in-hand. After all, Clinton and Obama were both seen as quite liberal , even though their records weren't that liberal. Perhaps some of the more liberal Democrats won't be seen as liberal this time around.

Further, ideology is far from the only thing that matters for election outcomes. Good personal stories matter, for instance. Sometimes the fundamentals are so clearly pointing in one direction that extreme candidates win anyway. Sometimes strong candidates can overcome being far left or far right, and otherwise weak centrist candidates don't win. Obama won two terms, despite being seen as very liberal. George W. Bush actually did better in 2004 than in 2000, even though voters viewed him as considerably more conservative the second time around. And Jimmy Carter's perceived moderate views didn't solve him from a bad economy in 1980.

Nominating a very liberal candidate is a risky bet, though, especially given the polling so far. The well-known left candidates are doing worse than the more moderate one. Nominating one of them certainly strays from the playbook that Democrats used in the 2018 election to take back the House.