The most expensive midterm campaign in U.S. history raced to a finish ahead of Tuesday’s election, as both sides braced for a possible split decision that would hand the House to Democrats and leave Republicans holding onto or expanding their Senate majority.

Partisans were preparing for the unexpected, though, two years after Donald Trump stunned the nation with his surprise win. As candidates, surrogates, outside groups, and the two parties frantically worked to turn out their voters, strategists for each party agreed the outcome will be determined by the composition of an electorate that’s showing signs of being larger than normal for a midterm year.

The verdict could dramatically alter the second half of Trump’s first term. If they win at least one chamber, Democrats have pledged to stifle the president’s agenda and start investigations into his finances, administration, and Russia’s meddling in the 2016 election.

Regardless of the outcome, there’s no doubt that this year’s election has generated an unusually high level of interest among voters. Typically, only about four in 10 eligible voters cast ballots in midterm elections, compared to six in 10 for presidential years.

Buoyed by strong feelings about Trump, pro and con, turnout could hit levels not seen in a half century, said Michael McDonald, a University of Florida professor who runs the United States Elections Project.

Read more in Bloomberg.