We are 280 days away from November's midterm elections. Welcome to the first look at CNN's Key Races - House ratings for 2018.

Washington (CNN) Democrats are eyeing the House majority, and for the first time since 2010 they stand a real chance of winning it.

According to CNN's new Key Race Ratings for the House, a large number of Republican-held tossup seats and a number of Republican-held seats that are leaning toward the Democrats mean control of the House is up for grabs.

Hoping to ride an energized base that's set on delivering President Donald Trump a major loss, the party knows just where to look for the 24 GOP-held seats Democrats need to flip in order to retake control of the House: the suburbs. Educated, diverse swing districts around Philadelphia, Minneapolis and Orange County, California, make up many of the most competitive races in CNN's first set of key House races.

Here's a look at three key categories of what look like the most competitive districts in November's midterm elections.

The Clinton-won districts

Republicans currently represent 23 districts where Hillary Clinton beat Trump in the 2016 presidential election. That list was Democrats' starting point in identifying pickup opportunities this year.

Four of these districts are suburban, won by Clinton and currently represented by Republicans who are departing the House -- qualifying them for "lean Democratic" ratings.

Rep. Martha McSally opted to run for the Senate seat of retiring Republican Jeff Flake in Arizona rather than face Democrat Ann Kirkpatrick -- fresh off a statewide campaign against Sen. John McCain -- in her Tucson-based 2nd District. In South Florida, Rep. Ileana Ros-Lehtinen, a fierce intra-party Trump critic, is retiring from her 27th District seat, which has moved rapidly leftward. (In the nearby 26th District, Rep. Carlos Curbelo isn't retiring but faces similar headwinds in what we're rating as a toss-up.)

And in Orange County, California -- which consists in part of four Republican-held districts that Clinton won in 2016 -- Reps. Ed Royce and Darrell Issa are retiring from districts that now lean Democratic. Jam-packed Democratic primary fields make it tough to predict how those races will shake out, but House Democrats' campaign arm -- knowing they would be prime targets -- opened a regional office in the area early in the 2018 cycle.

California Reps. Steve Knight, Mimi Walters, Dave Valadao, Dana Rohrabacher and Jeff Denham also represent Clinton-won districts, making their state perhaps the most important battleground on the 2018 House map. We rate Knight's and Rohrabacher's districts as toss-ups, while Denham's and Walters' seats lean Republican and Valadao's is likely to remain in GOP hands. If a Democratic "wave" is to form, the number of California pickups will be a useful gauge of its size.

Former Army ranger and attorney Jason Crow is a prized Democratic recruit on the east side of Denver, where he'll challenge Rep. Mike Coffman in what's sure to be an expensive bellwether race we rate as a toss-up.

Some of these Clinton-won seats won't be easy for Democrats to win, though. Republicans got a reprieve in New York's 24th District, where Syracuse Mayor Stephanie Miner, a Democrat, announced last week that she won't run against Rep. John Katko -- keeping Katko in the "likely Republican" category.

Open seats

So far, 50 of the House's 435 members have announced they are leaving Congress -- 35 Republicans and 15 Democrats.

Several of the Republicans, in particular, are opting to depart rather than face tough re-election battles. Eleven of those Republicans' seats made CNN's list of the most competitive races.

Monday brought another major retirement announcement: Rep. Rodney Frelinghuysen won't seek re-election in his northern New Jersey district. Democrats already had a prized recruit in Mikie Sherrill, a former naval helicopter pilot and federal prosecutor. The district was already moving in Democrats' direction: Mitt Romney won by 6 points in 2012, but Trump won by just 1 point in 2016. The race is a toss-up as Republicans scramble for a new candidate.

In addition to McSally, Royce, Issa and Ros-Lehtinen, Washington Rep. Dave Reichert's district south and east of Seattle is a major battleground. His retirement shows just how valuable these open seats are for Democrats; Reichert was re-elected in 2016 by 20 percentage points even though Clinton won the district by 3 points. He'd won by an even bigger margin in 2012, when President Barack Obama carried his district. With Reichert's departure, his district is now a toss-up.

The retirements of Reps. Dave Trott of Michigan, Frank LoBiondo of New Jersey and Lynn Jenkins of Kansas also broadened the competitive map for 2018. Trott and LoBiondo's districts are toss-ups, while Jenkins' district leans Republican.

Two more seats in Pennsylvania with retiring Republicans -- Rep. Charlie Dent's Lehigh Valley district and Rep. Pat Meehan's in the Philadelphia suburbs (another Clinton-won seat) -- fit this description. But there's a major question mark: The state's Supreme Court just ordered lawmakers to redraw Pennsylvania's congressional district lines by mid-February, which could completely change the state's political landscape before the 2018 midterm elections.

Democrats in Trump country

Republicans will mostly play defense in 2018, but there are a handful of seats where the party could poach Democratic seats -- and offset suburban losses.

Six districts where Trump delivered a strong performance in 2016 feature largely exurban and rural and white electorates.

New Hampshire's 1st District has flipped five times in the last six elections -- and with Democratic Rep. Carol Shea-Porter's retirement, the trend could continue. Nevada's 3rd District is another regular battleground, and with Rep. Jacky Rosen running for the Senate instead, it represents another potential battleground.

Another place to watch closely is Minnesota: Rep. Rick Nolan is up for re-election in an 8th District that swung from favoring Obama by 6 percentage points in 2012 to backing Trump by 16 in 2016. Southern Minnesota's 1st District swung nearly as heavily, and Democratic Rep. Tim Walz is running for governor rather than re-election.

These four Democratic-held seats all rate as toss-ups.

Methodology

CNN's 2018 House and Senate Race Ratings are based on a number of factors and data points. These include candidate recruitment, fundraising strength, districts' voting history, voter registration data by party and recent voting trends, as well as CNN's political reporting and analysis.

Glossary of key terms

Toss-up: The most competitive seats, where each party is heavily investing and each party has a roughly equal chance of winning.

Lean: Seats where one party has a slight advantage, but are solidly within the universe of competitive races.

Likely: Seats where one party has a clear advantage, but which could become competitive. During a true wave election, these are the seats to watch.

Wave election: Any election where one party makes significant gains in one or both chambers of Congress. The most recent wave election occurred in 2010, when Republicans picked up an astonishing 63 seats in the House and six in the Senate.

Pickup: A seat is picked up when it changes from one party to the other. In 2018, Democrats need 24 net pickups to win party control of the House, and two net pickups to win back the Senate.

Open seat: Any House or Senate seat that does not have an incumbent running for re-election. Seats open up due to members retiring, resigning, running for another office or dying.

Retiring: A member is retiring when he announces he will step down at the end of a term. Many members do this if they think they will lose re-election, or if they are term-limited out of committee chairmanships. Prominent members who are retiring at the end of the current Congress include California Republican Reps. Darrell Issa in the 49th District and Ed Royce in the 39th District and Democratic Rep. Ruben Kihuen in Nevada's 4th District.

Resigning: A member resigns when he steps down before the end of his current term. Recent examples of this include Michigan Democratic Rep. John Conyers, Arizona Republican Rep. Trent Franks and Pennsylvania Republican Rep. Tim Murphy.