The retirement of Senator Olympia J. Snowe of Maine is about as damaging to a party’s electoral prospects as these things get, turning a seat that Republicans were very likely to retain into one they will probably lose.

There have been some comparable cases in the recent past, but most were on the Democratic side, in particular the retirements of Senator Byron Dorgan of North Dakota and Senator Evan Bayh of Indiana in 2010, and the pending retirement Senator Kent Conrad, also of North Dakota, in this cycle.

Ms. Snowe’s retirement levels the playing field a bit. When we last took an overview of the Senate in December, I gave Republicans a 15 percent chance of losing Ms. Snowe’s seat. Even that 15 percent, however, was not an estimate of Ms. Snowe’s chances of losing in a head-to-head matchup against a Democrat; she remains popular in Maine and easily won re-election there in a difficult election cycle (2006). Instead, it was a hedge against the possibility that Ms. Snowe would retire or succumb to a primary challenge, precisely because the consequences of this would be so damaging to Republicans.

Maine, while idiosyncratic, is a blue-leaning state, enough that you would expect Democrats to have an edge there in an average election cycle in a contest between “generic” opponents.

The further complication for Republicans, however, is that Democrats are likely to have the better candidate, above and beyond their partisan advantage. Especially in a small state, the most natural ascendants to the Senate are members of the United States House of Representatives. Democrats occupy both of Maine’s House seats: Representative Chellie Pingree in the downstate First Congressional District, and Representative Mike Michaud in the Second Congressional District upstate.

Maine is quite homogeneous demographically; being elected from one of the two Congressional districts (as Ms. Snowe was repeatedly from 1978 to 1992) means that one will probably pass muster with voters in the whole state. Moreover, polls find that Ms. Pingree and Mr. Michaud are both reasonably popular, and both won re-election by double-digit margins in 2010, a very challenging cycle for Democrats otherwise.

Potential candidates to succeed Ms. Snowe face a tight deadline for doing so — they must gather enough signatures by March 15 to appear on the ballot. Both Ms. Pingree and Mr. Michaud have already begun doing so, according to the Bangor Daily News — as has a third Democrat, the former governor and U.S. Representative John Baldacci.

That certainly raises the prospect of a nasty primary fight: Senate seats haven’t come up often in Maine historically, and the state’s other senator, Susan Collins, a Republican, is fairly young and has survived previous attempts by candidates like Ms. Pingree to unseat her.

However, that is a good problem for Democrats to have compared to the one that Republicans face. Their potential candidates include the Maine secretary of state, Charlie Summers, and the state senate president, Kevin Raye. But they have less statewide stature than the Democrats do. Mr. Raye, for instance, had planned to run against Mr. Michaud in the Second District, but was thought by most observers to have only an outside chance of defeating him. The same presumably holds if that contest takes place over the whole state rather than just half of it.

A third Republican, Scott D’Amboise, was already on the ballot in a challenge to Ms. Snowe, but his Tea Party-inspired policy positions are probably too conservative for the state as a whole. Maine elected a Tea Party-backed Republican governor, Paul R. LePage, in 2010, but did so with only 38 percent of the vote in a four-candidate race. Mr. LePage’s approval ratings have wobbled between poor and average, however, and 2012 is unlikely to be a cycle as fruitful for the Tea Party as 2010 was.

One could argue, in fact, that Republicans should take their relatively stronger candidates in Maine, like Mr. Raye, and run them for the House; those seats could be more competitive if vacated by Ms. Pingree and Mr. Michaud, offering a lower reward but a considerably higher chance of success.

There is also the prospect that an independent candidate could be a factor in Maine, as they often are in the state. An independent, Eliot Cutler, nearly won the governorship there in 2010, and another independent, Angus King, was governor of the state before Mr. Baldacci and Mr. LePage. In some ways, Ms. Snowe was as much an independent as a Republican, with one of the most moderate voting records in the Senate; she cited the increased partisan rancor in the chamber as her reason to leave it.

Conceived as a two-candidate race, however, Democrats are heavily favored in Maine, perhaps having an 80 percent chance of picking up the seat in a head-to-head race against one of the Republicans. We will have an analysis of the effect this might have on the overall Senate math later this week.