At the start of this election cycle, historical trends suggested Democrats were likely to make gains in the House. Two years later, they appear to be on the verge of taking over the chamber. A few surprising developments helped them get there.

While President Donald Trump has attempted to deflect blame should Republicans lose House control Tuesday, he has certainly been a factor in this year’s contests. That’s typical for midterm elections — the president’s party has lost an average of 33 seats in 18 of the last 20 midterms.

It’s difficult to distill the fight for the House to a handful of developments, with individual candidates and differing dynamics playing out in dozens of races across the country. But there have been a few overlapping themes that help explain what has given House Democrats the edge this election season. 

Watch: How We Got Here — The Biggest Surprises in a Campaign Season for the Ages

1. Green wave

Early on it was clear this election was going to be expensive. Democratic candidates were quick to raise eye-popping figures fueled by small-dollar donations, but that could have been written off as what some fundraisers refer to as “love money,” or funds from family members and friends that dry up after the first quarter.

But Democratic donors kept giving, leading to a “green wave” that overwhelmed dozens of GOP incumbents, some of whom haven’t faced a competitive race in years. According to the Democratic Congressional Campaign Committee, more than 110 Democratic challengers outraised their Republican opponents in the third fundraising quarter that ended Sept. 30.

Corry Bliss, the executive director of the Congressional Leadership Fund, a super PAC aligned with House GOP leadership, wrote in a memo to donors last week that “the overall financial situation [for Republicans] remains alarming.”

2. Expanded battlefield

Fifty-nine GOP-held seats made the DCCC’s initial target list, but that grew to 101 by February of this year. Not all of those races are competitive, but the battlefield kept expanding throughout the cycle. The 25 Republicans in districts Hillary Clinton won in 2016 were natural targets, but Democrats also focused on districts that supported former President Barack Obama and flipped to Trump, as well as other traditionally Republican seats in the suburbs.

A slew of Republican retirements also opened up competitive seats for Democrats, further expanding the map. Republicans have a few open-seat targets in Minnesota and Nevada, where incumbents opted to run for higher office.

Democratic chances of picking up seats in Pennsylvania improved when the state Supreme Court imposed a new congressional map, making several districts more Democratic. Democrats also breathed a sigh of relief after the California primaries when they avoided a November shutout. (Under the Golden State’s top-two primary system, crowded fields could have split the Democratic vote, allowing two Republicans to advance to the general election.) But the national party did have to spend millions to make sure a Democrat made it out of those primaries.

Democrats also say their recruits in Republican-leaning districts, such as Utah’s 4th or New York’s 22nd, helped put tough races in play. In Utah, Salt Lake County Mayor Ben McAdams faces GOP Rep. Mia Love in a district Trump won by 7 points in 2016. And in New York, state Assemblyman Anthony Brindisi is taking on GOP Rep. Claudia Tenney in a district that backed the president by 16 points. 

3. Debates in Congress

Activity in Congress had a big influence on the campaign trail this cycle, beginning with last year’s failed GOP effort to repeal much of the 2010 health care law. Health care has dominated Democratic messaging, with candidates across the country highlighting the potential impact of the repeal effort. Republicans also worried that the failed attempt could deflate turnout among GOP voters, who for years had been promised that a Republican-controlled government would repeal the health care law. 

Republicans viewed passage of their tax overhaul as a major achievement that could help them retain House control. They’ve credited the tax law for the strengthening economy and expect it to pay dividends at the ballot box. But GOP leaders have had to remind their incumbents to tout the accomplishment on the campaign trail, and privately they wish Trump would have done so more often. At the same time, Democrats have been casting the overhaul as a tax break for the wealthiest Americans.

Republicans believe the recent Supreme Court confirmation battle over Justice Brett M. Kavanaugh could benefit their House candidates. One GOP strategist noted that the episode — in which Christine Blasey Ford accused  Kavanaugh of sexual assault when they were in high school — woke Republicans up, and could explain why some races, like GOP Rep. Rod Blum’s in Iowa’s 1st District, tightened in recent weeks.

But the high court fight also energized Democrats. And it threatens to tarnish Republicans in suburban districts, where GOP and independent women are already turned off by Trump.

4. Trump factor

A key question this cycle was whether some GOP voters supported Clinton in 2016 as a one-time rejection of Trump, or whether that anti-Trump sentiment would extend to congressional Republicans. The president remains unpopular in suburban districts so his campaign activity has been focused mostly on red states and districts.

Trump’s election also energized an unprecedented number of female Democratic House candidates as well as women who joined grassroots groups. Republican and independent women who disapprove of Trump are expected to be critical swing voters in GOP-held suburban districts.

5. GOP controversies

Republicans had some unwelcome developments, with two incumbents facing indictments ahead of Election Day. California Rep. Duncan Hunter was charged with misusing campaign funds, while New York Rep. Chris Collins was charged with insider trading. Both congressmen insist they’re innocent, but the controversies have made their deeply Republican districts more competitive than they might otherwise have been.

Another unexpectedly competitive district is Iowa’s 4th District, where GOP Rep. Steve King is facing renewed criticism for past racist remarks and statements supporting far-right personalities.

Some Republicans have faced ethics troubles, including Blum in Iowa who is under a House Ethics investigation for not disclosing his ownership of a company. Montana Rep. Greg Gianforte has faced some Democratic attacks after pleading guilty to assaulting a reporter ahead of last year’s special election. Democrats have also been attacking Virginia Rep. Scott Taylor for his staff’s involvement in forging petition signatures to get a third-party candidate (and potential Democratic spoiler) on to the ballot.

Simone Pathé contributed to this report.