Democratic presidential candidate Joe Biden speaks at Clinton Community College in Clinton, Iowa, on Wednesday. (Charlie Neibergall/AP)

Joe Biden attracted a lot of attention Tuesday after exchanging rhetorical fire with President Trump while in Iowa. But a far bigger news story was that Biden was in Iowa in the first place. That’s because his lead in Iowa polls is much smaller than his national margin, suggesting the Democratic front-runner is far from an inevitable nominee.

The Iowa caucuses are historically much more important to Democratic candidates than to Republicans. Only one of the state’s past four GOP caucus winners have gone on to win the nomination, as Iowa’s Republican electorate is much more conservative and evangelical Christian than the nation as a whole. But on the Democratic side, the Iowa winner has gone on to capture the nomination in each of the past four contested races.

That makes Biden’s slim lead more notable than his wide national lead. Candidates have been in and out of Iowa all year long. One, Marianne Williamson, has even moved there. Iowa Democratic voters have seen and heard much more from the contenders than almost any other state, and thus are much less likely to base their preferences largely on name identification. For Biden, that hasn’t been a good thing.

According to the RealClearPolitics polling average, Biden leads by five points in Iowa compared to 16 points nationally. This is not because of a lack of name ID, as Biden is as well known and nearly as well liked among Iowa Democrats as he is nationally. It’s just that more Iowa Democrats who do like Biden are picking someone else.

This puts Biden’s two-day foray into the Hawkeye State into context. He has thus far avoided the heavy schedule of in-person public events there that many other campaigns have had. This week’s events show he recognizes that he needs to pick up the pace to stay in the lead.

He also needs to play catch-up in building the ground game that Iowa experts say you need to win. Reports this week claim Biden is only now hiring the staff needed to identify potential supporters and turn them out to vote, historically a crucial part of winning a caucus state. He did not hire his first Iowa staffers until mid-May, months behind his competitors. There might still be enough time to get the nearly 1,800 volunteer precinct captains he needs for caucus night, but with 23 candidates, the supply of potential volunteers is limited.

The state’s ideological lean also hampers Biden. Sixty-eight percent of 2016 Democratic caucusgoers were liberals, according to the entrance poll. That’s bad news for Biden, whose national lead rests on moderate voters. There are simply many fewer potential Biden voters in the state than there are in the nation as a whole. This places organization at a premium, as it is crucial for him to turn out every last potential vote. But as we’ve seen, he’s already behind in that game.

Normally a candidate such as Biden could recover after an Iowa loss by winning the next state, New Hampshire. But that state is potentially even less friendly to him than Iowa. The 2016 exit poll showed Granite State Democratic primary voters were as liberal as Iowa caucusgoers. Biden will also likely compete with one or both of the candidates who serve as senators for neighboring states, Vermont’s Bernie Sanders and Massachusetts’s Elizabeth Warren.

Candidates from neighboring states typically do better in New Hampshire than they do elsewhere. Aside from Vermont’s Howard Dean, who lost to Massachusetts’s John F. Kerry in 2004, no Democrat from a state bordering New Hampshire has lost there since 1980, when Massachusetts Sen. Edward Kennedy lost to incumbent President Jimmy Carter.

Bill Clinton was the most recent Democratic nominee to lose both Iowa and New Hampshire, but the party was much different then. Many blue-collar whites still called the party home, and it was their votes — especially in Clinton’s native South — that lifted the Comeback Kid. Those voters have become Republicans since 1992, giving Biden little hope of pulling off a similar comeback if two early defeats taint him as a loser.

Biden still leads in Iowa, and there is still time for him to mount a serious campaign there. But watch the Iowa polls carefully. If Biden can’t win in Iowa, he’s unlikely to be able to catch up later.

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