US Democratic presidential candidates Bernie Sanders and Hillary Clinton are in a virtual tie in California, the country’s most populous state and one that until recently was strongly in Clinton’s favor, a new statewide poll has found.

Clinton is leading Sanders among likely voters, 46 percent to 44 percent, within the margin of error, according to a poll released Wednesday by the Public Policy Institute of California.

A poll by the same institute in March found Clinton with a lead of 48 percent to 41 percent over Sanders.

The survey came as both Sanders and Clinton have increased their campaign appearances and advertisements in California as the state's June 7 primary election approaches.

California is one of the most expensive states for television advertising, given its size and the number of media markets.

Sanders’ rallies have drawn big, enthusiastic crowds in many parts of California. His aides say he intends to spend nearly all his time in the state until the June 7 primary, underlying how much importance he has attached to a victory here as he tries to keep his campaign alive until the Democratic National Convention in July.

Even if Clinton gains enough delegates to capture her party’s presidential nomination, a loss in California would provide a sour and deflating end to her primary campaign.

It could also encourage Sanders to stay in the race through the party’s convention, at a time when many Democrats would be asking him to concede the race and join with Clinton in turning the party’s attention to defeating the presumptive Republican nominee, Donald Trump.

The new California poll showed that both Sanders and Clinton would beat Trump in a hypothetical November contest, though Sanders appears to have the stronger position.

Clinton is leading Trump among likely voters by 49 percent to 39 percent while Sanders would beat him 53 percent to 36 percent.

A nine-month FBI investigation into whether laws were broken as a result of a private email server kept in Clinton’s New York State home has overshadowed her campaign.

More than 2,000 emails sent and received by Clinton while working as the top secretary of state between 2009 and 2013 include classified information, which the government bans from being handled outside secure, government-controlled channels.