Barbara Mikulski was not pleased when she learned Senate Minority Leader Harry Reid planned to back a candidate to succeed her in next year’s Maryland Senate race at a time when many would-be contenders were still deciding whether to run.

Make clear, she told Reid, that the endorsement of Rep. Chris Van Hollen was the Senate leader’s alone and does not represent the official backing of the Democratic Party.

“There are other Democrats that are going to get into the race,” the retiring five-term senator said sternly last week, her voice rising. “We haven’t had a primary yet. We haven’t had a primary yet.”

Mikulski’s frustration is a reaction to a conscious strategy: Democratic leaders in the Senate see a path back to the majority, and they are actively working to clear the field for their favored candidates — quashing unpredictable and sometimes bloody primary fights before they even begin. Reid and his leadership team are betting that the party establishment still has enough sway in Senate primaries to overcome dissent — a far different calculation than Republican leaders who feel they must step lightly into primary elections for fear of igniting a conservative backlash.

In Ohio, for example, national Democrats have thrown the full weight of the Democratic Senatorial Campaign Committee behind former Gov. Ted Strickland’s attempt to win the party’s nod to take on Republican Sen. Rob Portman. Party officials worked unsuccessfully to muscle out 30-year-old Cincinnati Councilman P.G. Sittenfeld, who says he’s determined to stay in the race. In Florida, meanwhile, Reid is signaling that he favors Rep. Patrick Murphy, 31, and would like to keep the field clear for him, even as Rep. Alan Grayson indicated he’s increasingly interested in a bid.

Montana Sen. Jon Tester, chairman of the DSCC, said he has spoken with Murphy and that he could be a “great senator” — even though Murphy hasn’t yet officially launched a campaign. National Democrats are wary of a bid by Grayson, the outspoken liberal firebrand who they think would be a far weaker general election nominee and could hurt Murphy in the primary.

“I think in that particular case, I don’t know that a primary helps us,” said Tester.

In California, the establishment quickly embraced Attorney General Kamala Harris for the seat of retiring Sen. Barbara Boxer — before giving other potential standard-bearers a chance to emerge. New Hampshire Gov. Maggie Hassan is poised to clear the Democratic Senate field if she decides to try to unseat Republican Sen. Kelly Ayotte. And in Illinois, party elders are actively considering throwing their support behind Rep. Tammy Duckworth if she jumps into the race against GOP Sen. Mark Kirk, even as several House members and local officials are actively considering a run.

“I think she is the most formidable opponent, and it’s likely that many of the House members would acknowledge that,” said Illinois Sen. Dick Durbin, declining to say whether he would back Duckworth but adding that party leaders are looking for the strongest candidates who could survive general elections in competitive states.

Similarly, in the relatively few competitive governor’s races in 2016, Democrats have boasted that their strongest candidates — the attorneys general of North Carolina, Kentucky and Missouri — have uncontested paths to the party’s nomination. “We’d like to avoid primaries if possible,” said Connecticut Gov. Dannel Malloy, who will head the Democratic Governors Association in 2016.

Despite these moves, top Democrats are downplaying their involvement.

“I think we’re going to try to stay out of most primaries,” said Tester. “I don’t want to say never — and I don’t want to get myself boxed in on anything. … I’m thinking we’re going to take everything case by case. Personally, my inclination is, unless there’s good reason, let it play.”

Still, with 24 Republican seats up in 2016, and, so far, two Democrats retiring, Democratic leaders think that actively backing their favored candidates in select primaries is the best path to regaining the Senate majority next year. They’ve had some success so far, uniting their party behind preferred candidates in states like Missouri, where Secretary of State Jason Kander is an early underdog against incumbent GOP Sen. Roy Blunt; and Wisconsin, where the party is optimistic about former Sen. Russ Feingold’s chances against GOP Sen. Ron Johnson if the Democrat decides to mount a comeback bid.

Republicans, on the other hand, have been burned by conservative groups unhappy with the party’s choices in a number of key races over the past few cycles — including in Florida six years ago, when the National Republican Senatorial Committee lined up behind then-Gov. Charlie Crist. Driven by the support of Club for Growth and other right-leaning groups, Marco Rubio stormed to the top of the primary polls, and Crist dropped out of the primary to run as an independent. Though Republicans held the seat when Rubio won, the outsider’s rise was an embarrassment for the GOP establishment.

Since then, a series of intraparty GOP wars have roiled the Senate landscape, including several primaries in which far-right conservatives emerged over establishment-backed candidates — in states like Indiana, Delaware, Nevada and Colorado — but who later blew seemingly easy victories.

That’s why the NRSC likely won’t follow the Democrats’ tactics in crowded primaries this year. Asked on Capitol Hill last week if his group would endorse in open or Democratic-held seats, Mississippi Sen. Roger Wicker, chairman of the NRSC this cycle, shook his head “no.”

“Incumbents,” deadpanned Wicker, making clear that the NRSC would dip into primaries only when a sitting senator is on the ballot.

“It’s always proved to be harder for Republicans to clear the field all over the country than it has for Democrats, and I think that’s because Republicans are more individualistic by nature,” added Blunt, the first-term Missouri senator who serves in party leadership.

Unlike GOP primaries, where a handful of big spending conservative groups can influence races, the Democratic grass roots — including liberal groups like the Progressive Change Campaign Committee and Democracy for America, which is pleading with Sen. Elizabeth Warren (D-Mass.) to run for president — have yet to showcase their power against the party establishment during Barack Obama’s time in the White House.

“While the ability to raise money and implement smart get-out-the-vote tactics is important, nothing is more important than voters hearing a bold, popular, Elizabeth Warren-style message from Democratic candidates,” said Adam Green, co-founder of PCCC.

Democratic insiders aren’t too concerned about raising progressives’ ire if it means winning elections, but playing kingmaker isn’t without its risks.

Liberals and advocates for minority representation recoiled at Reid’s endorsement of Van Hollen in Maryland, which came a day before Rep. Donna Edwards, who is African-American, announced she is challenging Van Hollen for the nomination and as a slew of other Democrats — including Reps. Elijah Cummings and John Sarbanes, and former Lt. Gov. Kathleen Kennedy Townsend — were still weighing bids.

“I found it surprising — with two qualified black veteran members of the House, a former lieutenant governor whose father and uncle served in the Senate, and also a sitting member of the House whose father served in the Senate — that the minority leader would make an endorsement before everyone had made their intentions known,” said Kimberly Woodard, a Democratic lobbyist formerly for Walmart and a board member of the Congressional Black Caucus PAC.

Similarly, Democrats’ quick endorsement of Harris in California infuriated Latino leaders, who view the open seat as one of their first opportunities to elect a standard-bearer for the community.

In a brief interview last week, Reid defended his endorsement of Van Hollen: “He’s my friend, always has been, I’ll support him on whatever he does.”

But Reid’s decision to back Van Hollen was followed quickly by a statement of neutrality from the DSCC.

“Sen. Reid is entitled to his own personal opinion, but I asked him to make it clear that was not an official position of the Democratic Senate Campaign Committee,” Mikulski said.

Reid’s endorsement was countered on Thursday by the pro-Democratic women’s group EMILY’s List, which said it was throwing its support and fundraising help to Edwards.

And when it suits them, national Democrats occasionally back contested primaries when the front-runner isn’t in their good graces.

One state in which Democrats have been tacitly encouraging a primary is Pennsylvania, where establishment animosity toward Joe Sestak, who barely lost a 2010 Senate race to now-Sen. Pat Toomey, has kept the party on the sidelines as he seeks a rematch. Democratic insiders in Pennsylvania have expressed open contempt for Sestak, who burned bridges when he bested the late Sen. Arlen Specter in the 2010 Democratic primary. Yet, Sestak is sitting on more than $1.5 million in his campaign account and has no obvious challengers for the nomination, and there is an active debate among party elders on whether to clear the field for the former congressman.

Asked whether he would prefer a clear field for Sestak, Bob Casey, Pennsylvania’s senior Democratic senator, said with a smile, “You know, it’s only 2015.”

“I don’t pay any attention to it,” Sestak said of the insider grousing.

Former Pennsylvania Gov. Ed Rendell said heartburn among party leaders didn’t hurt Sestak in 2010 and will do “even less damage” in 2016. “Maybe the party bosses don’t like him, but I think the active Democrats like Joe Sestak,” he said.

In Ohio, Sittenfeld entered the Senate race forcefully, lining up a powerful team of former Obama campaign operatives and raising what allies hinted was more than a half-million dollars in the first few weeks of his candidacy.

But many in the party expected him to fold when Strickland entered the race, deferring to the ex-governor. When he didn’t, the DSCC came out for Strickland, and insiders began worrying that a fierce primary might weaken the eventual nominee against Portman. Sittenfeld shrugs off the notion and says he’s not going anywhere.

“The endorsements that matter most are the ones from Ohioans,” said Sittenfield, who acknowledged that he spoke with Tester just days before the committee backed Strickland.

Still, Democrats in Washington say the fewer bloodier primaries in 2016, the better their chances will be to retake the Senate.

“There will not be a primary” on the Democratic side in Missouri, the state’s senior senator, Claire McCaskill, declared.