Four of this year’s key seats — from the St. John Valley to the Portland suburbs — were held by Democrats before 2014, when they flipped and helped give Republicans the 20-15 edge that they hold now. An analysis of voter registration data, this year’s candidate slate and past election results suggest those seats could be prime for Democratic takebacks.

Democrats must gain three seats to take the majority and that could happen in 2016, another higher-turnout presidential year. But Republicans say they’re banking on individual candidates who match their diverse districts.

Democrats could win as many as seven seats held by Republicans, who don’t look to have many pick-up opportunities.

— Sen. Peter Edgecomb, R-Caribou, isn’t running for re-election in his district, where registered Democrats have an edge of 17 percentage points on Republicans. Former Sen. Troy Jackson, D-Allagash, beat Edgecomb in 2010 and 2012 and is running against Caribou City Councilor Timothy Guerrette, a Republican. BDN rating: Strong Democratic

— Sen. Scott Cyrway, R-Benton, is running against Rep. Henry Beck, D-Waterville. Cyrway beat a Democratic incumbent easily in 2012 and could do it again, but Democrats have an 11-point edge on Republicans here.BDN rating: Leans Democratic

The BDN rated five other Republican seats as toss-ups:

— Senate President Mike Thibodeau, a Republican from Winterport, is running against Democrat Jonathan Fulford of Monroe again after a recount was needed in their 2014 race. The district leans Republican by less than 1 percentage point.

— Sen. Linda Baker, R-Topsham, will have a rematch with 2014 opponent Eloise Vitelli, a Democrat from Arrowsic. Vitelli won the seat after a 2013 special election but then lost it to Baker in 2014. The district leans Democratic by 3 percentage points.

— Sen. Rodney Whittemore, R-Skowhegan, is running against House Majority Leader Jeff McCabe, D-Skowhegan. Whittemore, a third-term senator, won more than two-thirds of votes in 2014, but he’s never had an opponent with McCabe’s political experience and the district only leans Republican by about 1 percentage point.

— Sen. Amy Volk, R-Scarborough, beat an incumbent Democrat in 2014 in Maine’s most even district by party registration, with just 24 more Republicans than Democrats. She’s facing Scarborough Town Councilor Jean-Marie Caterina, a Democrat with an unsuccessful 2012 House run under her belt.

— Sen. Earle McCormick, R-West Gardiner, isn’t running for re-election. Former Democratic U.S. Senate candidate Shenna Bellows of Manchester and Gardiner City Councilor Terry Berry are running for their party’s nomination, while Gardiner City Councilor Maureen Blanchard and retired U.S. Navy Rear Adm. Bryan Cutchen of West Gardiner are running in the Republican primary. The district leans slightly Democratic.

The Democratic seat most likely to flip may be that of Sen. Chris Johnson of Somerville, who’s running against former Sen. Dana Dow, R-Waldoboro. The district leans Republican by 5 percentage points. But Johnson beat Dow in a 2012 special election, has defended the seat twice and may have an easier path in 2016.

Can Republican Senate candidates keep “a bubble” around their races against the Trump and LePage effects?

Both Thibodeau and Senate Minority Leader Justin Alfond, D-Portland, praised their parties’ slates for this year’s election as among the best they’ve seen. How they’re defined by each other during the campaign will be key.

Nationally, Democrats are trying to tie Republicans to the divisive Donald Trump, and in Maine, legislative Democrats often have used LePage as their main campa ign foi l, though he won re-election in 2014 and may not be the liability that Democrats have framed him as.

But this crop of Senate Republicans often lined up against LePage, working with Democrats to pass a budget over the governor’s veto in 2015, which drew the governor’s ire and led to Thibodeau becoming one of the Legislature’s key peacemakers.

Alfond was policy-focused when previewing Democrats’ arguments against Republicans, mentioning their opposition to Democrats’ proposal for millions more in school funding than they achieved this year under a compromise with Republicans, among other issues.

“They have to defend their records,” Alfond said. “The governor’s not up for election this year.”

Lance Dutson, a Republican strategist, said it’ll be “a harder slog” for the party to keep a majority this year, though he said the Senate’s divide with LePage has given them “an opportunity to create bubbles around their races” and form unique rapports with voters in key districts where the governor and Trump are less popular.

“Assuming that those swing districts also don’t like Trump at the top of the ticket, there’s going to be some serious legwork for them to do on the ground,” Dutson said. “But that being said, they’re all really good candidates.”

Thibodeau said nobody’s ever told him they voted for a legislative candidate based on national factors and “the voter wants Maine to have a strong economy,” which he said Senate Republicans have championed.

“We have some tremendous people who are willing to serve,” he said, “and I cannot wait until voters get an opportunity to meet these people.”

But this will be the last Legislature to deal with LePage before he leaves office in early 2019, so a Democratic win in the Senate would foil his agenda and deliver a blow to his legacy.