Senate Democrats need to gain four seats — five if a Republicans wins the White House — to win back the majority in 2016. Their top priorities are competing in more than a half-dozen blue and purple battlegrounds. The Democrats believe they can capitalize on the presidential race, boosting party turnout in states like Wisconsin, Illinois, Ohio, Pennsylvania, North Carolina, New Hampshire and Florida, which Republicans are defending.
Republicans, meanwhile, feel like they can go on offense against Democratic incumbents in Nevada and Colorado.
Maryland doesn't appear to be moving swiftly in the direction of those more competitive contests as a result of Mikulski's decision. The nonpartisan Cook Political Report, which scrutinizes Senate races and judges their relative competitiveness, opted to leave Maryland in the "Solid Democratic" column, the safest category.
"The results of last year's gubernatorial contest notwithstanding, Maryland is a solidly blue state and the Republican bench is especially weak," wrote the Cook Report's Jennifer Duffy explaining the decision. "The race here is going to be the Democratic primary."
The open Democratic race could entice ambitious rising stars and well-established politicians to jump in. There are no shortage of names party officials and strategists would feel very comfortable nominating.
The names being bandied about, reports The Post's John Wagner, include Reps. Chris Van Hollen, Elijah E. Cummings, Donna F. Edwards, John Delaney, John Sarbanes, former governor Martin O'Malley and Baltimore Mayor Stephanie Rawlings-Blake.
Republicans say that intense Democratic competition could leave the eventual nominee bruised and could force that person to tack farther left than would be ideal ahead of the general election. On the heels of Republican Larry Hogan winning the governor's race last fall, the GOP says it will field an elite Senate contender who can capitalize on an intra-party fight on the other side.
"While Democrats get ready for a bloody primary, we will have a top recruit waiting for whoever emerges," said National Republican Senatorial Committee spokeswoman Andrea Bozek in a statement.
Rep. Andy Harris, former governor Robert L. Ehrlich Jr., retired surgeon Ben Carson and Lt. Gov. Boyd Rutherford are some of the Republican names being mentioned as possible candidates. None are seen as politicians with an ability to instantly upend the race.
Mikulski is vacating her seat in a very Democratic state in a presidential election year. The last time a Republican presidential candidate won in Maryland was 1988. Hillary Rodham Clinton or whoever Democrats nominate for president could have major coattails in Maryland.
"I am confident that in November 2016, we will elect a new Democratic senator who will fight for Maryland every day and make Barbara Mikulski proud," said Democratic Senatorial Campaign Committee Chairman Jon Tester (Mont.) in a statement.
Mikulski is the second senator to announce retirement plans so far this year. The other is California Sen. Barbara Boxer. Her seat is also expected to remain in Democratic hands.
Retirements can instantly put seats in play or force parties to expend resources that would otherwise go to other states. In the case of Mikulski's seat, it seems Republicans have some potential to make things interesting. But they are not there yet.