House Democrats are heading into 2018 feeling the wind at their backs and sensing their best chance in years to retake the lower chamber.

Midterm elections for first-term presidents are typically grim for the party in control of the White House. And with President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s approval rating hovering in the 30s — an historic low in the modern era — the Democrats are increasingly optimistic they can mine the White House turmoil to energize their base, attract disheartened Republicans and pick up the 25 seats they'll need to win back the Speaker's gavel after eight years in the minority.

The Republicans scored a huge win this month with the passage of their tax-code overhaul — a victory Trump and GOP leaders maintain will lend them momentum heading into the new year.

ADVERTISEMENT

But the GOP has struggled on other legislative fronts, and where the Republicans see their tax win as a renewal of strength, the Democrats sense desperation. On the day after the tax vote, House Minority Leader Nancy Pelosi Nancy PelosiPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Trump signs largely symbolic pre-existing conditions order amid lawsuit MORE (D-Calf.) suggested the Republicans were forced to race the bill to the finish line this year because they know they're in trouble next November.

“They’re acting like people who know that their power is short-lived,” Pelosi said as Congress was leaving town.

The Democrats have their own challenges heading into next year’s midterms. The district map, much of which was rewritten by GOP state houses following the 2010 census, leans heavily in favor of the Republicans keeping their majority. And Democrats have their own internal divisions to contend with amid growing calls for the top leaders, all in their mid-seventies, to step out of power to allow a younger generation of rising stars to assume the role of face of the party.

But despite those tests, the leading election prognosticators increasingly consider the House up for grabs next year. Here are some of the factors affecting the odds.

History

Historically, the party controlling the White House has been walloped in midterm elections. Researchers at Sabato’s Crystal Ball, an election handicapper based at the University of Virginia, have crunched the numbers, finding that the president’s party has lost House seats in 36 of 39 midterm cycles dating back to the Civil War, with an average loss of a whopping 33 seats.

The trend is especially pronounced when the president is unpopular. In 2006, when the Democrats beat expectations to win back the chamber, President Bush’s approval rating was just 39 percent, versus 57 percent who disapproved, according to the RealClearPolitics average of national polls, putting him 18 points underwater. President Obama, similarly, was almost 11 points underwater heading into the 2014 midterms, when the Republicans gained 13 seats.

Trump’s struggles have been even more pronounced. Since the inauguration, when voters were evenly split in their views of the incoming president, Trump has become ever more unpopular, with just a 39 percent approval rating in the latest RealClearPolitics average, versus almost 57 percent of those polled who think he’s doing a poor job.

Retirements

House Republicans are facing an exodus of veteran lawmakers, some of whom have already left Capitol Hill and others who plan to step away at the end of the term. The reasons vary; 10 are seeking another office, 14 are retiring, four announced early resignations and four others left to join the Trump administration. All told, 32 Republicans who began the year in the conference — more than 13 percent of the group — won’t be back in the chamber in 2019, and the number could rise in the months to come.

“They’ve got members in some cases they don’t particularly want to have leave,” said Rep. Zoe Lofgren Zoe Ellen LofgrenBusiness groups start gaming out a Biden administration Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence DHS opens probe into allegations at Georgia ICE facility MORE (D-Calif.), referring to the handful of Republican retirees who will soon be term-limited out of committee chairmanships.

The departures leave the Republicans scrambling to fill dozens of open seats. And while many of those are in deep-red districts the GOP is sure to keep, others are in more contested regions carried last year by the Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE, including seats currently held by outgoing Reps. Ileana Ros-Lehtinen Ileana Carmen Ros-LehtinenEx-Florida GOP congresswoman under federal investigation: report 'Trump show' convention sparks little interest on K Street Shalala to face Salazar in Florida rematch MORE (R-Fla.) and Dave Reichert David (Dave) George ReichertWashington Rep. Kim Schrier wins primary Mail ballot surge places Postal Service under spotlight Bottom Line MORE (R-Wash.), as well as that held by Rep. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R-Ariz.), who’s widely expected to enter the 2018 race to replace retiring Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R-Ariz).

The Policy Factor

Democrats were clobbered in the 2010 midterms, losing 63 seats and Pelosi’s Speaker’s gavel in what was widely considered a referendum on the adoption of ObamaCare eight months earlier. Fast forward seven years, and the Democrats see stark parallels in the Republicans’ approval of tax-reform legislation.

Republicans have touted their achievement as an economic boon to Americans of all incomes, vowing to use it as a campaign tool next year.

“This bill will help ease the cost of living for millions of Americans who feel left behind,” said Matt Gorman, spokesman for the National Republican Congressional Committee.

“We will run on tax reform and win on tax reform in 2018.”

But Republicans have struggled to sell their message to voters. An NBC News/Wall Street Journal poll conducted this month found that just 24 percent of respondents think the GOP’s bill is “a good idea” (versus 41 percent who say it’s a bad plan), while just 22 percent say it will benefit people of all classes (versus 63 percent who think it’s primarily designed to benefit wealthy Americans and corporations).

Those numbers play right into the message from Democrats, who are warning that millions of middle-class Americans will see a tax hike, while many of those receiving cuts would see the benefits erased in the form of higher health costs.

“This is fairly a big opening. The door is open,” Pelosi said. “Most people know that they're not getting any tax break in this bill.”

Polls

Heading into the year, the Democrats thought that if they approached a double-digit lead in the generic ballot they’d have a good shot at taking back the Speaker’s gavel next November. Their advantage is currently 13 points, according to the most recent RealClearPolitics average, and a CNN poll this month had them up by 18 points.

“I’ll take half of that, nine, that's enough to win the Congress,” said Pelosi, adding that she doesn’t “place a whole lot of confidence in all of these double-digit polls.”

Despite Pelosi’s cautious approach, the Democrats have seen those numbers translate into surprising election victories in recent weeks. In November, they celebrated a long string of local and state victories coast to coast, including the much-watched Virginia governor’s race that was widely viewed as a referendum on Trump’s first year in office.

More recently the Democrats were encouraged by Doug Jones’s shocking upset in the special election to replace Sen. Jeff Sessions Jefferson (Jeff) Beauregard SessionsRoy Moore sues Alabama over COVID-19 restrictions GOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs MORE (R-Ala.), Trump’s attorney general, in a state that hasn’t elected a Democratic senator since 1992.

Republicans have been quick to downplay the significance of the Alabama contest, chalking up their defeat to a flawed candidate in Roy Moore, who faced a series of allegations that he’d romantically pursued and engaged in sexual encounters with teenagers decades ago.

Still, Trump’s unsuccessful push for Moore has only emboldened the Democrats, who see new limits to the power of the president’s populist sway, even in deep-red states like Alabama.

Rep. Ben Ray Luján (N.M.), the head of the House Democrats’ campaign arm, said the Trump “backlash” has helped the party build “the largest battlefield in a decade.” His office, the Democratic Congressional Campaign Committee, recently announced that it raised almost $7 million in November — up from $4.5 million two years ago.

“What’s going to be most important is for our candidates to be connecting in their districts … and telling that story,” he said.