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(CNN) —  

The top Democratic candidates best President Donald Trump in national head-to-head matchups, according to a new poll from Quinnipiac University out Tuesday.

With the general election still more than a year away, former Vice President Joe Biden leads the pack, with the largest advantage over Trump. The poll shows 53% of voters said they would support Biden and 40% said Trump. Biden and Trump held side-by-side events in Iowa on Tuesday, both using their platforms to insult the other one. Biden called Trump an “existential threat” to the country and Trump said Biden was a “dummy.”

The gap is smaller but still a distinct advantage for Sens. Bernie Sanders (up 9 percentage points), Kamala Harris (+8) and Elizabeth Warren (+7). Others candidates, such as Sen. Cory Booker and Mayor Pete Buttigieg, scrape by the margin of error, leading Trump by 5 percentage points each.

Biden holds a 30-point lead among independents: 58% back him, vs. 28% for Trump. Trump’s best showing among independents in this poll is 34% against Warren.

Most of the Democratic candidates took to Iowa last weekend, with primary season off to the races. Trump has a few primary challengers, but no polls have shown them as seriously competitive with him.

Numbers this far out from a general election are hard to read, and could change between now and November 2020. They should be thought of as a snapshot in time rather than a predictor of the future.

Biden’s edge over Trump has increased slightly since 2015, when Quinnipiac first asked about their head-to-head matchup. At that point, 49% said they would support Biden and 37% said Trump. The same is true of Sanders, going up from 44% to Trump’s 41% in August 2015 to 51% over 42% now.

But the general election is still a far way off, and anywhere between 4% and 7% of registered voters said they weren’t sure who they would vote for, depending on the matchup.

A Fox News poll found similar results in mid-May when asking about the general election, with Biden leading Trump by 11 percentage points, Sanders leading the President by 5 percentage points and Warren by 2.

In the primaries, Biden still dominates the race, according to the Quinnipiac poll, but his lead is diminishing. Three-in-10 Democrats and Democratic-leaning independents said they will support the former vice president in the Democratic primary, while 19% went for Sanders and 15% for Warren.

Biden hit 38% in the Quinnipiac poll taken immediately after he announced his run for the White House, and he has lost 8 percentage points since then. But that’s expected of candidates after their announcements and the initial bumps they receive in polling.

Sanders originally lost some ground and has gained it back, while Warren’s support has increased since March, from 4% to 15%.