For Republicans, the 2018 House playing field is a lot like a game of Whack-a-Mole: everywhere they turn, new problems keep popping up in surprising places. In January, we rated 20 GOP-held seats as Toss Ups or worse, including three leaning towards Democrats. With today's changes, we now rate 37 GOP-held seats as Toss Ups or worse, including ten leaning towards Democrats.

Republicans are relieved that state Sen. Troy Balderson appears to have eked out a win in Ohio's 12th CD special election last week. But a new round of polls shows several more GOP incumbents, including Reps. Mimi Walters (CA-45) and Tom MacArthur (NJ-03) highly vulnerable. Their seats, along with Rep. Robert Pittenger's open NC-09, move from Lean Republican to the Toss Up column.

On the bright side for Republicans, a handful of their battle-tested incumbents appear to be defying the "blue wave" in Democratic-leaning seats. Recent campaign polling shows Reps. David Valadao (CA-21), Carlos Curbelo (FL-26), John Katko (NY-24) and Will Hurd (TX-23) with impressive initial leads in districts Hillary Clinton carried. This week, Curbelo moves from Toss Up to Lean Republican.

Rating Changes:

CA-45: Walters | Lean R to Toss Up ←

FL-26: Curbelo | Toss Up to Lean R →

NJ-03: MacArthur | Lean R to Toss Up ←

NC-09: OPEN (Pittenger) | Lean R to Toss Up ←

PA-10: Perry | Likely R to Lean R ←

Bottom Lines

CA-45: Mimi Walters (R) - Inland Orange County: Irvine, Mission Viejo

Toss Up. This wealthy southern Orange County district voted for Hillary Clinton 49 percent to 44 percent after voting for Mitt Romney by 12 points in 2012. But today, Walters, a former stockbroker and who has won her past races comfortably, is locked in a very competitive contest against UC-Irvine law professor Katie Porter, a consumer advocate and a protege of Elizabeth Warren.

Republicans thought they caught a break when Porter won the right to face Walters in June, edging out more centrist UC-Irvine colleague Dave Min in the top-two primary. Porter grew up in Iowa and comes across as more of a Midwestern populist than a coastal suburbanite. After the 2008 housing crisis, Porter was appointed by then-state Attorney General Kamala Harris to be the state's mortgage settlement monitor.

But the burden is on Republicans' to make the race a referendum on Porter and Elizabeth Warren instead of President Trump. And that could be hard to do, especially after Walters voted for the GOP's healthcare bill and tax bill, which included changes to the SALT deduction that could hit plenty of wealthy 45th CD voters hard. Democrats will argue Walters has "voted with Donald Trump 99 percent of the time."

Republicans attack Porter for expressing support for single-payer healthcare and sidestepping taking a position on the gas tax repeal ballot initiative, which they hope raises GOP enthusiasm in a year when no GOP candidates are competitive statewide. But Porter has already raised $2 million with the help of EMILY's List and will likely be as well-funded as the incumbent.

Of the four GOP-held seats in Orange County, Walters remains the likeliest to survive. But it's clear she's in for a very competitive race. She joins the Toss Up column.

FL-26: Carlos Curbelo (R) - South: Homestead, The Keys, The Everglades

Lean Republican. Curbelo should have no business surviving in a suburban Miami seat that voted for Hillary Clinton 57 percent to 41 percent in 2016. But the Republican hasn't been shy about his support for immigration reform and a carbon tax, or calling the president's remarks in Helsinki "unacceptable" and "deeply alarming," and private polls continue to show him in surprisingly good shape for reelection.

Democrats say Curbelo's vote for the GOP healthcare bill proves he's a secret Trump ally hiding behind a moderate facade. And, they believe they'll benefit from nominating someone other than disgraced former Rep. Joe Garcia: fundraising consultant Debbie Mucarsel-Powell, a fundraising consultant and former FIU official who lost a state senate race in 2016 but has raised $1.6 million for this race.

Mucarsel-Powell believes her personal story of losing her father to gun violence in Ecuador will resonate in a district not far from Parkland High School. But she's also not the most polished or dynamic candidate and could struggle against Curbelo in a debate setting. It's still a competitive race, but Curbelo starts out well ahead and the burden is on Democrats to link him to the president.

NJ-03: Tom MacArthur (R) - South central: Burlington, Toms River

Toss Up. By the numbers, Republicans should have the edge here: this retiree-heavy seat voted for President Trump 51 percent to 45 percent, and MacArthur took 59 percent in 2016. But a new Monmouth University poll shows MacArthur leading Democratic nominee Andy Kim just 41 percent to 40 percent, more than a month after MacArthur launched his first general election ads.

The wealthy insurance executive's greatest exposure is his voting record: he was one of the architects of last year's failed GOP healthcare bill. He was also the only member of New Jersey's delegation to vote for the GOP's tax cut package, despite a reduction of the SALT exemption that could impact New Jersey especially hard. Kim, who has already raised $2 million, will pound those votes over and over again.

MacArthur's path to survival will involve spending personal money to attack Kim, a young former aide to Gen. David Petraeus and ISIS adviser in the Obama White House. Kim grew up in the district but only recently moved back home to run - and his resume opens him up to charges of elitism. Two weeks ago, MacArthur launched an ad calling Kim a "tax cheat" over a $72,000 tax break on his DC condo.

MacArthur hasn't faced an opponent nearly as well-funded as Kim in his last two races, and has never had to run at a time Republicans control everything in DC. Kim will carry his Burlington County base, but MacArthur is counting on Ocean County's GOP retirees to pull him through. If Republicans lose this race, it's possible they could be reduced to one of the state's 12 seats. MacArthur moves to the Toss Up column.

NC-09: OPEN (Pittenger) (R) - Southwest: Charlotte suburbs

Toss Up. Evangelical minister Mark Harris unseated Rep. Robert Pittenger 49 percent to 46 in the May 8 GOP primary. The former senior pastor of First Baptist Charlotte had come within a point of beating Pittenger in 2016, and this year urged voters to "drain the swamp" by tossing out a "career politician" whose real estate business had been under FBI investigation. But now, Harris's main obstacle to the House is a serious Democrat.

The 9th CD was dramatically redrawn in 2016, which helped diminish Pittenger's incumbency advantage. Under the new lines, it stretches from the Charlotte suburbs to parts of Fayetteville and President Trump carried it 54 percent to 42 percent. But it's got enough ancestrally Democratic rural counties that Iraq veteran Dan McCready, who was recruited by fellow Marine Seth Moulton, has a plausible path to victory.

McCready grew up in Charlotte and joined the Marines after 9/11. After leaving the military, he earned an MBA from Harvard, became a management consultant and started a solar energy company that installed solar farms throughout the state. Republicans will portray McCready as an elitist who profited from President Obama's "taxpayer-funded solar sham," but he didn't have a primary and banked $1.8 million at the end of June.

Republicans now insist they're better off with Harris, a charismatic movement conservative, than the staid politician Pittenger. Harris is well-connected from his days as president of the Baptist State Convention and led the fight for a traditional marriage amendment in 2012. But whereas Pittenger was personally wealthy, Harris emerged from the primary with just $295,000 on hand at the end of June.

Democrats are already attacking Harris over a 2013 sermon in which he appeared to question whether it was healthy for women to make careers the top priority in their lives, in order to drive a wedge between suburban independents and the GOP base. But the biggest problem for Republicans is McCready's $1.8 million to $295,000 cash edge, which means Harris may not be able to neutralize those attacks.

The fundamentals of the district ought to give Harris an advantage, but it looks like GOP outside groups will need to come to his rescue. McCready's financial muscle pushes this race into the Toss Up column.

PA-10: Scott Perry (R) - Central: Harrisburg, York

Lean Republican. At least several Keystone State Republicans are concerned that Perry, a three-term Freedom Caucus member, remains unprepared for a fight in a Harrisburg district that was redrawn in February to be much more competitive and is 41 percent new to him. Under the old lines, President Trump won Perry's seat by 22 points. But under the new lines, Trump won it by just 52 percent to 43 percent.

In the May primary, Democrats nominated pastor and retired Army officer George Scott with 36 percent. Some Democrats believe his military background and status as a political outsider matches up well against Perry, an Iraq War veteran and former state legislator. Scott outraised the incumbent $241,000 to $188,000 in the most recent fundraising period (Perry still had more cash on hand, $553,000 to $207,000).

If need be, Perry could probably use Scott's primary ad, in which the Democrat throws rifles onto a bonfire while calling for expanded background checks, against him. But Perry has never had to run in a competitive general election, and this seat is now much more suburban. If Gov. Tom Wolf and Sen. Bob Casey win comfortably at the top of the ticket, this race could fly under the radar and feel the effects.