Democratic senators gearing up for competitive re-elections tend to have whiter staffs, according to a Roll Call analysis of data released by Senate Democrats.

Sen. Jon Tester of Montana, who finds himself in a race rated Tilts Democratic by Inside Elections with Nathan L. Gonzales, has a staff that is 93 percent white. West Virginia Sen. Joe Manchin III, also in a Tilts Democratic contest, was just behind him, at 92 percent.

Those numbers mirror the demographics of their home states, where 86 percent and 92 percent of people, respectively, are white.

Sen. Heidi Heitkamp of North Dakota, whose race is rated Tilts Republican, comes in at 86 percent.

This trend is not necessarily caused by the political leanings of the state. Several factors affect senators’ hiring decisions. A stronger link, for example, exists between the racial makeup of a senator’s home state and the diversity on his or her staff.

Reporter’s Notebook: Vulnerable Democratic Senators Have Less Diverse Staffs

Sen. Angus King, a Maine independent who caucuses with Democrats, has the whitest staff of any senator that reported this year: 95 percent. His re-election race is rated Solid Democratic.

The Pine Tree State has the whitest population of any state — 93 percent, according to Census Bureau estimates from 2016.

Diversity in King’s office has decreased since last year, when 11 percent of his staffers were non-Caucasian.

Hawaii Sen. Brian Schatz has the most diverse office in the Senate Democratic Conference.

Non-Caucasians make up 72 percent of his staffers. His office’s diversity comes mainly from Asian or Pacific Islander staffers, who account for 59 percent.

Schatz also had the most non-Caucasian staffers in 2017, with 66 percent.

There are some notable outliers, though. Not all Senate staffs closely align with the demographics of their state.

California Sen. Dianne Feinstein, who faces a fellow Democrat in November in her bid for a fifth term, has a staff that is 32 percent non-Caucasian — slightly lower than the average for the conference. That’s despite the fact that Feinstein represents one of the least white states in the country.

California’s junior senator, Kamala Harris, by contrast, has the second least Caucasian office among Senate Democrats, with a 66 percent nonwhite staff. Her office is diverse across the board at 26 percent Latino, 26 percent black or African-American, 14 percent Asian or Pacific Islander and 3 percent Native American, Native Hawaiian or Alaska Native.

New Jersey Sen. Cory Booker goes against the trend in the opposite way: His staff is less white than those of other senators from states with similar diversity levels. He has the third-most nonwhite staff, with 63 percent.

Last year, Tester had the least diverse office, with 93 percent white staffers, and Manchin was second-least diverse, with 91 percent.

The women issue

Sens. Chris Van Hollen of Maryland and Maggie Hassan of New Hampshire are tied for the most female staffers this year, both at 67 percent, according to the data collected through the Senate Democrats’ Diversity Initiative. Both senators have increased their numbers since 2017, when Van Hollen had 64 percent female staffers and Hassan had 55 percent.

Last year, New Jersey Sen. Robert Menendez had the most male staffers at 60 percent. This year, no Democratic Senate office cracked that number.

Indiana Sen. Joe Donnelly, who’s in a Toss-up race, has the most male staffers at 58 percent, followed by Minnesota Sen. Amy Klobuchar and Rhode Island Sen. Jack Reed at 56 percent.

While some offices may seem to be lagging behind when it comes to diversity, publicly releasing this data is considered a breakthrough in Congress. There is no rule that requires congressional offices to provide data on the racial and ethnic makeup of their staff for the public record.

Lorenzo Olvera, the Diversity Initiative director who reports to Senate Minority Leader Charles E. Schumer, first collected the data last year. He and his team, which includes a deputy director, research aide and interns, sent out a voluntary survey, which the full conference agreed to complete.

The initiative dates back to 2007, but the data collection and establishment of the so-called Rooney Rule — encouraging offices to consider at least one minority candidate when interviewing for an open position, just as the NFL does in filling coaching vacancies — was a Schumer mandate.

Least represented demographics

Staffers of Middle Eastern or North African origin are the least represented demographic among Senate Democratic offices, according to the data. Twenty-four of the 49 offices have no Middle Eastern staffers.

Sen. Gary Peters of Michigan and Sen. Christopher S. Murphy of Connecticut are tied for the most staffers from Middle Eastern or North African backgrounds, with 8 percent each.

Twenty-three offices have no Native American, Native Hawaiian or Alaska Native staffers — all of whom the data reports in one group.

The Hawaii senators — Schatz and Mazie Hirono — have the most from this group on staff, with 15 percent and 13 percent, respectively. No other office breaks 10 percent.