Washington (CNN) The results of Tuesday's special election in western Pennsylvania offer an ominous sign for Republican hopes of defending the House this fall. CNN is making changes to its rating of 17 House races -- all of them in the direction of the Democrats.

Democrat Conor Lamb is poised to deliver a stunning upset in a district that President Donald Trump won by 20 percentage points less than two years ago. That performance, combined with 2017 wins in Alabama, Virginia and New Jersey, and strong showings in previous special elections, all reflect the headwinds Republicans are facing in November's midterm elections.

In light of Tuesdays results, there are several districts that now appear more favorable for Democrats this year, with a similar mix of candidate quality, district composition and, in some cases, concerns about the Republican incumbent, or they represent open seats.

Of the 12 races CNN now rates Lean Democratic, 8 are currently held by Republicans. Of the 21 races CNN rates Tossup, 19 are held by Republicans. Democrats need to pick up 23 seats in order to take control of the House in November.

IL-12: Democrats have tried to recruit Brendan Kelly to run before, but he's held off until now. The St. Clair County prosecutor hails from a voter-rich part of the district and outraised GOP Rep. Mike Bost in the final quarter of 2017. Trump won this district by 14 points, but Barack Obama carried it by two points in 2012. This district has a history of voting Democratic, so it should offer an even better opportunity for Democrats than PA-18. Race moves from Lean Republican to Toss-up.

MI-08: This is another Midwest contest where Democrats have cleared the field for a top recruit, former intelligence official Elissa Slotkin. She's challenging Rep. Mike Bishop, who won this district that includes Detroit exurbs and Lansing by 17 points in 2016. Trump carried the district by six points, and Romney won it by just three points in 2012. Slotkin also raised $140,000 more than Bishop the last three months of 2017, though still trails in cash on hand by about $260,000. Race moves from Lean Republican to Toss-Up.

MI-11: Republican Rep. Dave Trott's retirement from this district -- home of Chrysler and many of the American auto giants' workers -- left it vulnerable to a Democratic takeover. Trump carried it by 4 points in 2016, but this district has several characteristics that suggest a strong shift is coming. Among them: It's suburban, and it's dominated by the auto industry, which means Trump's new tariffs on steel and aluminum imports (which make vehicles' components more expensive) likely won't be popular. Democrats still have a crowded primary to sort out, but they'll be watching the Detroit suburbs closely. Race moves from Toss-Up to Lean Democratic.

MN-03: Republican Erik Paulsen won this district comprising the suburbs west of the Twin Cities by 14 points in 2016 -- but Hillary Clinton bested Trump here by nine points. It also went narrowly for Obama in 2012. Democrats are touting Dean Phillips, the former CEO of his family's distillery company and former co-owner of Talenti Gelato. He's keeping close in fundraising, trailing Paulsen by about $90,000 in the fourth quarter of 2017, but has a cash-on-hand deficit of more than $900,000. Race moves from Lean Republican to Toss-Up.

NJ-02: With the retirement of GOP Rep. Frank LoBiondo, this seat is high up on the target list for Democrats. The party has put state Sen. Jeff Van Drew on its Red-to-Blue list. The moderate Democrat, a gun rights supporter, does have a primary against Tanzie Youngblood. There is a crowded field on the GOP side. This district, which includes Atlantic City and some Philadelphia exurbs, went for Obama by eight points and Trump by four. Race moves from Toss-Up to Lean Democratic.

NJ-07: A once-crowded primary field for Democrats has started to narrow, and Tom Malinowski is the beneficiary. The former assistant secretary of state in the Obama administration saw two primary opponents drop out last month and endorse him -- that after he more than doubled GOP Rep. Leonard Lance in fundraising during the final quarter of last year. Hillary Clinton won this north central New Jersey district by a single point in 2016 while Lance won it by 11 points. Race moves from Lean Republican to Toss-Up.

NC-09: This is a tough district for Democrats. But so was Pennsylvania 18. Democrat Dan McCready is a top recruit who has outraised GOP Rep. Robert Pittenger for three consecutive quarters and has $600,000 more in the bank. Pittenger also faces a primary challenge from Mark Harris, who he defeated in the 2016 GOP primary by a single percentage point. The district includes South Charlotte and shares a border with South Carolina -- a similar dynamic to what Democrats were facing in PA-18. Race moves from Likely Republican to Lean Republican.

NC-13: Freshman GOP Rep. Ted Budd won this north central North Carolina district by 12 points in 2016 as Trump was carrying it by nine. Democrats are lining up behind Kathy Manning, a philanthropist and former lawyer, who crushed Budd in fundraising in the fourth quarter of 2017 and has roughly $220,000 more cash-on-hand. Race moves from Likely Republican to Lean Republican.

PA-17: The dust hasn't settled from Tuesday's special election, but regardless of the result it shows Democratic strength in southwestern Pennsylvania. This newly-drawn district is a much more competitive seat for GOP Rep. Keith Rothfus to defend. Assuming Conor Lamb decides to run in this district that includes suburbs west of Pittsburgh, it will be one of the most closely-watched contests come November. Race moves from Lean Republican to Toss-Up.

VA-07: This district that includes Richmond suburbs has been trending Democratic in recent years. Trump's performance here dipped from Romney's showing four years earlier. Republican Ed Gillespie won it by less than four points in last year's gubernatorial contest. GOP Rep. Dave Brat stunned the political world when he knocked off Eric Cantor in the GOP primary in 2014. Now he's poised for a challenge of his own from one of two Democrats who outraised him at the end of last year -- former CIA officer Abigail Spanberger or Marine Corps veteran Dan Ward. Race moves from Likely Republican to Lean Republican.

WI-01: This is House Speaker Paul Ryan's district, and it's likely to stay that way in November. Ryan can raise as much money as he wants, is universally known in his district and won't make the mistake of sleeping on Democratic challenger Randy Bryce. But Bryce has some things going for him, too. He's branded himself effectively (the mustachioed ironworker and Army veteran's Twitter handle is @IronStache). His longshot bid against Ryan will make him a small-dollar fundraising star. And southeastern Wisconsin is actually friendlier territory to Democrats than southwestern Pennsylvania was. Race moves from Solid Republican to Likely Republican.

In addition to those moves, we are shifting FL-06 and TX-21 from Safe Republican to Likely Republican. In Florida, GOP Rep. Ron DeSantis is leaving his seat to run for governor -- and Democrats believe former ambassador Nancy Soderberg gives them a chance in this open contest. While in Texas, Rep. Lamar Smith's retirement creates an open seat with May runoffs for Republicans and Democrats to determine who will face-off for this district that includes parts of Austin and area north of San Antonio. Smith won the district by 21 points; Trump won it by 10.

Given the current national environment we are also moving several Democratic-held seats to more favorable positions. Two open seats -- NV-03 and NH-01 -- go from Toss-Up to Lean Democratic. Democrats are happy with the candidates running in those districts, and in the case of New Hampshire, no Republican candidate seems to have caught fire.

Two other seats with Democratic incumbents seeking reelection move from Lean Democratic to Likely Democratic: FL-07 and NJ-05. For Republicans, the name of the game in 2018 is defense, not offense, with two open Democratic seats in Minnesota representing the GOP's best pick-up opportunities.

Those are today's moves, but there are a number of other races we're watching closely to see whether they inch in Democrats' direction. In Utah, Ben McAdams (challenging GOP Rep. Mia Love in the 4th District) has run well with Mitt Romney on the ballot before. In central Arkansas' 2nd District, Clarke Tucker was Democrats' dream recruit to take on two-term Rep. French Hill. Two Ohio races -- the 1st District, where Aftab Pureval is challenging Rep. Steve Chabot in what used to be solidly Republican Cincinnati suburbs, and the 12th District, where voters will replace retired Rep. Pat Tiberi in an August special election -- bear watching. In KY-06, we're waiting to see whether Lexington Mayor Jim Gray or veteran Amy McGrath wins the primary. And in KS-02, Democratic challenger Paul Davis is worth keeping an eye on, in part because he's already made clear that, like Lamb, he's opposing Pelosi. Democrats had struggled to recruit a strong challenger to Rep. David Valadao in CA-21, a Central Valley district, businessman TJ Cox — who'd been running in the 10th District against Rep. Jeff Denham — changed his mind and said he'll move from Modesto back home to Fresno and run against Valadao in this district Hillary Clinton won by 15 percentage points.

Correction: This story has been updated to correct a rating change in the text for IL-12 and in the graphic for NJ-02.