WASHINGTON — The Rev. Jesse Jackson, a key ally of Senate candidate Andrea Zopp, stepped up pressure on the Democratic Senatorial Campaign Committee on Wednesday for endorsing her rival, Rep. Tammy Duckworth, D-Ill.

The controversy developed after the DSCC decided to take sides in the contested March 2016 Illinois Democratic primary in which the winner takes on Sen. Mark Kirk, R-Ill.

Defeating Kirk is one of the top priorities of the Dem committee, led this campaign cycle by Sen. Jon Tester, D-Mont.

In a letter to Tester obtained by the Chicago Sun-Times, Jackson wrote, “to the chagrin of many loyal Democrats, we have read in the press that Representative Tammy Duckworth has been endorsed by your committee without a phone call or the proper vetting conversation.”

“Ms. Zopp is in the community she represents. She deserves respectful treatment,” Jackson said.

In closing, the civil rights leader wrote, “We appeal to you for further conversation.”

The committee’s endorsement brings to a campaign direct spending on behalf of a candidate and access to significant fundraising opportunities — a reason Zopp and her allies are trying to get the DSCC to reconsider and stay on the sidelines.

Duckworth was at the committee’s retreat on Martha’s Vineyard last weekend, attended by the organization’s best donors and where Sen. Al Franken, D-Minn., hosted a fundraiser for her.

The National Republican Senatorial Campaign Committee gleefully circulated a statement Zopp released last week that she was “saddened and shocked” over the DSCC endorsement, accusing the political shop of “total insensitivity to Chicago, African-American women and the democratic process.”

Jackson told me on Wednesday, “We are concerned” about the DSCC “not calling or considering or vetting” Zopp.

Zopp’s pollster, Ben Tulchin, contacted a regional DSCC political staffer when Zopp started talking about a Senate run. She was invited to reach out, I am told.

But a get-together never resulted — neither Zopp nor the DSCC pushed for a meeting to happen. The DSCC does not have a formal endorsement process.

Last week, Justin Barasky said before endorsing Duckworth they met with others considering an Illinois Senate bid, including Reps. Cheri Bustos, Bill Foster and Robin Kelly, who is African-American.

Baraksy said the DSCC “kept the lines of communication open but Andrea Zopp did neither.”

Anyway, the commmittee is not standing down.

On Wednesday, Barasky said: “The DSCC is proud to have endorsed Tammy Duckworth and stand with her throughout the campaign because she is the best candidate to beat a senator like Mark Kirk, whose racist and sexist comments have repeatedly put him in hot water.”