WASHINGTON — The Iowa caucuses are going to give one Democratic presidential hopeful a boost as the primary season kicks off on Feb. 3. Which candidate? That will most likely depend greatly on what the caucus electorate looks like on that day. And in recent election years, that has varied a lot.

The caucuses always arrive as something of an unknown and they can be predictive, not just on who the nominee will be but who this cycle’s Democratic voters are. How tuned-in are voters this cycle? Is there one group that seems especially engaged?

The turnout and voter composition in the past few Iowa caucuses have revealed a lot about the Democrats in their respective election years.

In 2004, Sen. John Kerry won on his way to capturing the Democratic nomination. The turnout was good, but not stellar, with an estimated 124,000 people turning out to have their voices heard. The win for Kerry, who was the Democratic establishment candidate over upstart Howard Dean, may have been an early sign that voters were not especially excited, but looking for a safe bet to beat then-President George W. Bush.

In 2008, Sen. Barack Obama surprised many when he won the caucuses in a year with record Democratic turnout — an estimated 239,000 Iowans came out to vote that year. The numbers were a sign that Democrats were indeed fired up and Obama was a big reason why. He went on to the nomination and a big win in November.

In 2016, Hillary Clinton narrowly edged Sen. Bernie Sanders in a race that was closer than most expected with a solid 171,000 people turning out to vote. The numbers suggested that Sanders had deep support among a group of voters who would turn out and who could drag the nominating fight on for months. The numbers were right.

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And those electorates looked different on a range of points. Let’s start with age.

In 2004, the caucus electorate was older, with 68 percent above the age of 45. The 2008 electorate was the youngest of the three, with only 60 percent in the 45-plus group. And the 2016 electorate sat in between the extremes, with 64 percent in that 45-and-older group.

Those numbers may come as a surprise to some. Bernie Sanders’ voters are often thought of as a younger, college-age demographic, but he came within a hair of winning in Iowa in 2016 with an electorate that was not especially young. It’s a reminder that Sanders also does fairly well with older, rural Democrats.

And the lower percentage of younger voters in Iowa may have presaged the challenge Democrats would have with those voters in 2016. Voters 18-44 made up a smaller share of the 2016 electorate in November and Clinton didn’t do nearly as well with them as Obama did in 2008.

If there is a larger trend in the Iowa numbers, it is on gender. The state’s Democratic caucus goers have grown more female in the last few open primary fields.

In 2004, there was an 8-point difference in the percentage of the caucus electorate that was female and male — 54 percent were women and 46 percent were men. But in 2008 and 2018, the difference was far greater. Women made up 14 percent more of the Democratic caucusgoers than men, 57 percent were women and 43 percent were men in both caucuses.

The female caucus numbers in 2008 and 2016 look like more evidence of the growing gender gap between Democrats and Republicans. Women have grown to become a solid Democratic voting group in the last 15 years.

But the most notable difference in the electorates show up in ideology, particularly in 2016.

In 2004, 56 percent of Iowa’s caucus goers said they were ideologically liberal. The number was slightly lower in 2008, but roughly the same, 54 percent.

In 2016, however, liberals dominated the caucus. More than two-thirds of the electorate, 68 percent, said they were very or somewhat liberal. That may help explain how Sanders did so well in the state — a specific part of the Democratic voting base was motivated. And that same group seemed to turn out in other states as well, powering Sanders through the primary calendar.

Were those big 2016 numbers for liberals a sign of a larger ideological shift in the Democratic Party or more of an Iowa/2016-specific event? The size and shape of the Democratic caucus electorate eight days from now will give us an indication.

If it looks like 2016 — solid, but not great, turnout with a big liberal bent — that’s a good sign for Sanders or maybe Sen. Elizabeth Warren. If it’s more like 2008 — massive and more moderate — candidates such as former Vice President Joe Biden, Pete Buttigieg or Sen. Amy Klobuchar may hold the advantage.

Regardless, don’t just watch who wins in Iowa on Feb. 3. The larger 2020 story may be as much about who comes out to vote.