This April 3, 2014 file photo shows then-Senate Majority Leader Senator Harry Reid (D-NV) as he listens during an event on Capitol Hill in Washington. (Brendan Smialowski/AFP/Getty Images)

It would be easy to conclude that Democrats are in better shape to hold the Senate seat from Nevada in 2016 after Sen. Harry M. Reid’s decision, announced Friday, not to run for another term.

Reid, after all, rarely won his races by anything but the skin of his teeth, and his role as the leading national Democrat in the Senate tainted his image badly in his home state. He won in 2010 in no small part because Republicans nominated the single worst candidate to run in that election — or maybe any recent election.

Yet, the most fundamental truth of politics is that it’s very hard to beat an incumbent.

Reid’s presence in the race was already making it difficult for Republicans to recruit a top-tier challenger. While Gov. Brian Sandoval (R) was never going to run — and I would be stunned if he reconsidered that decision after Reid’s announcement — the prospect of facing Reid and his political machine was decidedly daunting for any up-and-coming Republican.

That major impediment now has been removed. Sure, Reid is making moves — endorsing former state attorney general Catherine Cortez Masto, for one — that suggest he will be engaged in the race to become his successor. But there is a big difference between running against Reid and running against a candidate Reid supports. One is scary; the other is less so. (Sandoval proved that point when he drubbed Rory Reid, the senator’s son, to win the governor’s race in 2010.)

The field — for both parties — is still taking shape. Rep. Joseph J. Heck (R-Nev.), a well-regarded pol, has to be seen as more likely to run for an open seat than against Reid. Adam Laxalt, the newly elected Republican state attorney general, who is the grandson of longtime senator Paul Laxalt (R-Nev.) and the son (via an extramarital affair) of former senator Pete Domenici (R-N.M.), also would have to be more interested in a Reid-less race.

The point is that Reid was a known (and feared) commodity among Nevada Republicans. Cortez Masto, assuming she runs, does not inspire those same feelings of trepidation. And that’s a good thing for Republicans eyeing the state as a pickup.

Below are the 10 Senate seats considered most likely to switch parties in 2016. The No. 1 race is — you guessed it — the most likely to switch.

10. Indiana (Republican controlled): Sen. Daniel Coats’s unsurprising retirement announcement last week puts this seat on the line for now, in large part because it’s an open seat. But just because Democrats pulled off an upset here in 2012 doesn’t mean they’ll be favored to do so again. They got a lot of help last time when Richard Mourdock made his controversial comments about rape. Barring a run from former senator Evan Bayh (D) and his $9.9 million campaign account, Republicans should be favored in this red state.

9. North Carolina (R): Sen. Richard Burr voted last week like a senator who was very conscious of his reelection campaign in 2016, but his seat isn’t considered a top Democratic target just yet. The major question here is whether Democrats can secure a big-name nominee to run alongside Attorney General Roy Cooper (D) in the governor’s race.

8. Ohio (R): Democrats got their man when former governor Ted Strickland decided to run. But Cincinnati City Council member P.G. Sittenfeld shows no signs of getting out of the primary and could hurt Strickland by making it a new-vs.-old dynamic (Strickland is 73; Sittenfeld is 30). Strickland remains a strong favorite, given his fundraising capacity and the depth of endorsements he has won from Democrats. But it’s not entirely clear how vulnerable Sen. Rob Portman is.

7. New Hampshire (R): Nothing will happen in this race until Gov. Maggie Hassan (D) makes up her mind about challenging Sen. Kelly Ayotte. That decision isn’t likely to come until midsummer, when the state budget fight wraps. If Democrats don’t get Hassan, this race will drop in competitiveness. Another interesting twist to keep an eye on: Ayotte will almost certainly be in the mix to be the vice-presidential pick for Republicans.

6. Florida (R): This seat is as highly ranked as it is because (1) Sen. Marco Rubio probably needs to vacate it to run for president — which he seems likely to do — and (2) Rep. Patrick Murphy (D) just got into the race. The young congressman, though, might face a contested primary against controversial Rep. Alan Grayson, who could pull him to the left (and who would be a disastrous nominee if he somehow won the primary).

5. Colorado (Democratic controlled): With Reid’s retirement, the only Democratic incumbent running in 2016 who is in any danger of losing is Michael Bennet. Bennet, like Reid, won an unlikely victory in 2010 — a year that was disastrous for Democrats nationally. He again will be a major target, but it’s not clear yet whom Republicans will field as their candidate. The most talked-about option is Rep. Mike Coffman, but who knows whether he will run? Republicans will find someone, but, as 2010 proved, they will need someone better than Ken Buck to beat Bennet.

4. Pennsylvania (R): Democrats have made no secret of their discontent with the idea of former congressman Joe Sestak, who lost to Sen. Pat Toomey in 2010, as their nominee again. But they don’t appear to have any alternatives worth mentioning — even as Sestak has launched his campaign by walking across the state. We’ve long held that Toomey’s political skills are underrated, but this will be a very tough race, given presidential-year turnout in the Democratic-leaning Keystone State.

3. Wisconsin (R): A recent poll from Democratic-leaning automated pollster Public Policy Polling showed former senator Russ Feingold (D) leading Sen. Ron Johnson 50 percent to 41 percent. Three Democratic members of Congress, though, trailed Johnson by six to eight points. Feingold seems increasingly likely to run.

2. Nevada (D): See above.

1. Illinois (R): Sen. Mark Kirk is endangered in a blue state, and it doesn’t help that former congressman Joe Walsh, a tea party favorite, is considering a primary challenge. On the Democratic side, keep an eye on Rep. Tammy Duckworth in the weeks ahead.

Aaron Blake contributed to this report.