WASHINGTON (AP) — Bernie Sanders' mathematical path to winning enough delegates for the Democratic nomination is rapidly disappearing.

The Vermont senator now needs 57% of the delegates that are still available to take on President Donald Trump. A candidate needs to collect 1,991 delegates to win the nomination.

Sanders acknowledged the hurdles he faces after former Vice President Joe Biden took 70 more delegates than Sanders during Tuesday's contests. With more votes being tabulated in Washington state, Biden has a more than 150-delegate lead, which has been growing in March.

“Last night obviously was not a good night for our campaign from a delegate point of view,” Sanders told reporters Wednesday in Burlington, Vermont.

Both delegate allocation math and voting history show how unlikely it is for Sanders to overtake Biden.

When he faced Hillary Clinton in 2016, Sanders only got 55% or more of the delegates in nine of the 32 states or territories that have upcoming contests. And those nine states or territories have only 299 delegates at stake. The other 23 states and territories that have yet to vote have 1,829 delegates up for grabs and in 2016, he only got 41% of the delegates from those locales.

Four years ago, Sanders won only 48% of the last 2,020 delegates in the final primaries and caucuses. And in 2008, Clinton got only 45% of the remaining delegates after coming out of Super Tuesday behind Barack Obama.

While there are still more than two dozen delegates to be allocated from California’s ongoing vote count, trends would give both Biden and Sanders 10 more with the rest going to other candidates. That means that Sanders is unlikely to gain much more on Biden from the California delegates that have not yet been allocated, according to The Associated Press delegate count.

The intricate arithmetic of how delegates are won makes it even tougher for Sanders. Delegates are given out proportionally to candidates who get at least 15% of the vote. And in most cases, that’s both Biden and Sanders and no other candidates, except statewide in Mississippi, where Sanders just missed the mark statewide.

When there are an even number of delegates up for grabs in a district or statewide and a race is fairly close, the two candidates often split the delegates. It takes a big margin in votes to pick up an extra delegate.

That makes it rather difficult for the candidate trailing, in this case Sanders, to catch up to the front-runner. After next Tuesday, more than half of the delegates up for grabs will be in these districts with an even number of delegates.