By Brian Johnson, Center for Responsive Politics

Sen. Elizabeth Warren (D-Mass.) stands to gain from former U.S. housing secretary Julián Castro’s endorsement as Castro had most donors in common with Warren compared to other Democratic candidates. 

On Thursday, Castro suspended his campaign for president, and on Monday, he endorsed Warren in her bid for the presidency. Castro is the second former Democratic primary candidate to endorse a current candidate. 

In a tweet, he said: “Today I’m proud to endorse @ewarren for president. Elizabeth and I share a vision of America where everyone counts.”

Castro and Warren have about 29,000 donors in common, OpenSecrets research indicates, which is about 7,000 more than Castro has in common with any other candidate. 

These donors have raised $900,000 and $2.2 million for Castro and Warren respectively. Of all these common donors, only 46 donated the maximum amount of $2,800 to Warren, meaning that Castro’s exit from the race could allow most of his donors to continue support Warren.

In addition to sharing so many donors, Castro and Warren have two of the highest ratios of women donors. Castro also had a high proportion of Iowa women as donors. Almost 64 percent of Iowa donors to Castro’s campaign through quarter three of 2019 were women. Warren had the third highest proportion of female Iowa donors at about 53 percent. 

Nationally, almost 57 percent of Castro’s donations came from women. About 51 percent of Warren’s money came from women. The next closest to Warren is Sen. Amy Klobuchar (D-Minn.), who received nearly 49 percent of her money from women.    

Castro’s exit and endorsement could be a much-needed boost for Warren as she has dropped in the polls nationally and in Iowa since mid-October. Warren fell from a high of around 23 percent nationally to around 15 percent, according to FiveThirtyEight’s average of polls. Warren now trails former Vice President Joe Biden, Sen. Bernie Sanders (I-Vt.) and former South Bend Mayor Pete Buttigieg in Iowa. Warren raised less money in the fourth quarter 2019 than the third quarter, and she raised about $13 million less than Sanders. 

Buttigieg has also received a significant number of donations from donors who also donated to Castro. About 22,000 donors raised close to $734,000 for Castro and about $1.65 million for Buttigieg, which indicates that not all donors may switch to supporting Warren. However, only 40 percent of Buttigieg’s donors are women, so he may have more difficulty than Warren in appealing to Castro’s donors.

Castro raised $7.5 million through the third quarter, but he spent most of it and only had about $670,000 in available funds. Castro did not qualify for the Democratic debates in November and December. Despite having met the minimum number of donors, Castro was not polling high enough to qualify. 

A NBC News/Wall Street Journal poll from December showed Castro polling at 1 percent nationally against his Democratic presidential primary opponents. 

Brian Johnson, Center for Responsive Politics