Sen. Barbara A. Mikulski (D-Md.) holds up her BlackBerry as she responds to a tweet she just received during a hearing in June. (AP Photo/Charles Dharapak)

Maryland Lt. Gov. Anthony G. Brown’s gubernatorial campaign plans to unveil Sen. Barbara A. Mikulski as its latest supporter at an event Sunday, according to several people familiar with the planned endorsement.

The backing of Mikulski (D-Md.), who rarely chooses sides in Democratic primaries, is scheduled to be announced in Silver Spring two days before Douglas F. Gansler, Brown’s chief rival for the 2014 nomination, plans to formally launch his campaign for governor.

Brown (D) advertised the event in an e-mail to supporters last week, promising “an exciting campaign announcement” Sunday at noon. But his aides have remained coy about what will be announced. Justin Schall, Brown’s campaign manager, declined to comment Tuesday, as did an aide to Mikulski.

Brown’s plans were confirmed, though, by three people familiar with the upcoming endorsement who requested anonymity because they were not authorized to speak on behalf of Brown or Mikulski.

Brown, who launched his campaign in May, has already rolled out a series of high-profile backers, including Gov. Martin O’Malley (D), House Minority Leader Steny H. Hoyer (D-Md.), Rep. Elijah E. Cummings (D-Md.), state Senate President Thomas V. Mike Miller Jr. (D-Calvert) and Baltimore Mayor Stephanie Rawlings-Blake (D).

Aides to Gansler, Maryland’s attorney general, have said he will get his share of endorsements after he enters the gubernatorial race Sept. 24. But Gansler has instead focused in the early stages of the contest on rolling out policy proposals, on issues including domestic violence prevention and manufacturing incentives.

“Hopefully, Senator Mikulski will finally prod Anthony Brown, after 130 days as a candidate, to add an issues section to his Web site and offer up ideas to solve important challenges, like the out-of-control Baltimore prison scandal or the sky-high foreclosure rate, which are both occurring on his watch,” said Gansler strategist Doug Thornell.

A third Democrat, Del. Heather R. Mizeur (D-Montgomery), is also pursuing the nomination.

Mikulski, a Baltimore native, has long been Maryland’s most popular politician, according to public opinion polls. This year, she became chairwoman of the powerful Senate Appropriations Committee.

Mikulski, the longest-serving woman in U.S. Senate history, was first elected to the chamber in 1986. She had previously served five terms in the U.S. House representing Maryland’s 3rd district.

Gansler, a former Montgomery County state’s attorney, plans to kick off a 17-stop announcement tour in Rockville on Sept. 24. He plans to name a running mate in October, his advisers say.

On Tuesday, Brown’s campaign said it would focus in the coming week on promoting Brown’s record as lieutenant governor and as a legislator. In October, Brown will outline a series of policy proposals, starting with his plans for early childhood education, the campaign said.