Democratic primary voters' opinions of Sen. Bernie Sanders have improved, but both former Vice President Joe Biden and Sen. Elizabeth Warren saw their standing decline in a new Insider poll.

On January 22, Insider polled 426 likely Democratic voters whether their opinions of the 2020 candidates they knew of had improved or declined in January.

Overall, Sanders' standing among Democratic voters improved by 5.5 percentage points on net. Warren's net standing declined by 11.1 percentage points in January, and Biden's declined by a net 16.8 percentage points.

Sanders is consolidating the progressive vote, digging into Warren's support, and posing a direct challenge to Biden.

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In the weeks leading up to the Iowa caucuses, Democratic primary voters' opinions of Sen. Bernie Sanders have improved, but both former Vice President Joe Biden and Sen. Elizabeth Warren saw their standing decline in a new Insider poll.

For the past several months, Insider has been conducting a series of SurveyMonkey Audience polls to gauge Americans' opinions on 2020 Democratic primary. You can download every poll here, down to the individual respondent data. (Read more about how the Insider Democratic primary tracker works here).

On January 22, Insider polled 426 likely Democratic voters whether their opinions of the 2020 candidates they knew of had changed in January. Specifically, we asked, "Over the past three weeks, has your opinion of any of the following people declined or improved?"

The candidate with the biggest net improvement was the billionaire Michael Bloomberg, who has already spent over $270 million of his own money on the campaign. He had a net improvement of 10.5 percentage points, with 27.3% of respondents saying their view him improved compared with 16.8% who said it had declined.

Andrew Yang's standing among Democratic voters improved by a net 9.3 percentage points in January.

Sanders and Tom Steyer saw net improvements of 5.5 and 5.6 percentage points, respectively, during the month.

Sen. Amy Klobuchar and former Mayor Pete Buttigieg saw modest net improvements of 3.3 and 2.4 percentage points in their standing.

Warren's net standing declined by 11.1 percentage points in January, with 33% of respondents saying their opinion declined and 22% saying it improved.

More Democratic voters said their view of Biden changed for the worse, with 35% saying it declined in January and just 18% saying it improved, a net shift of 17 percentage points for the worse.

It's impossible to know exactly which factors are driving Sanders' rise and Biden's and Warren's declines in favorability among likely Democratic voters, but Sanders appears to be peaking at the right time in the week before the Iowa caucuses and New Hampshire primary.

Sanders is consolidating the progressive vote, digging into Warren's support, and posing a direct challenge to Biden. The former vice president is a Democratic frontrunner but still has to contend with a somewhat splintered moderate coalition, with some voters still deciding between him, Buttigieg, and Klobuchar in Iowa.

While the state of the field is still volatile and any number of outcomes are possible, Sanders' campaign is in its strongest spot in months, so much so that it's reportedly spooking establishment Democrats that he could win the nomination without a concerted effort to stop him.

Not only did he bring in an eye-popping $34.5 million in 2019's last fundraising quarter, but his performance in national, Iowa, and New Hampshire polls has dramatically increased over a relatively short period of time.

Since the beginning of January, Sanders went from 19% to 23% in Real Clear Politics' average of national Democratic primary polls. And in Iowa specifically, Sanders went from polling at 19% on average at the beginning of the month to 25% at the end of January, leading Biden by 3 percentage points on average.

Biden and his family have been in the spotlight in recent weeks amid the impeachment trial of President Donald Trump in the US Senate, but he is still performing well both in national polls and in later voting states including Nevada and South Carolina.

SurveyMonkey Audience polls from a national sample balanced by census data of age and gender. Respondents are incentivized to complete surveys through charitable contributions. Generally speaking, digital polling tends to skew toward people with access to the internet. SurveyMonkey Audience doesn't try to weigh its sample based on race or income. A total of 1,083 respondents were collected January 22, 2020, a margin of error plus or minus 3 percentage points with a 95% confidence level.