AUSTIN — Of the 51,249 Texans who cast ballots Tuesday on the first day of early voting, more than half voted in the Democratic primary.

The total number of voters from the 15 counties with the most people registered is high for a midterm year. In 2016, a presidential election year, 55,931 Texans voted on the first day of early voting for the primary. But in the last midterm election in 2014, only 38,441 Texans voted on the first day.

Even more surprising is the turnout among Democrats. Since the last midterm election, the party saw a 51 percent increase in first-day early voting turnout, while Republicans saw a 16 percent increase.

In Dallas County, 4,023 people voted in the Democratic primary, an increase over the 2,587 people who voted in 2014 and the 2,908 in 2016.

Dallas County in-person early voting, day 1:

Dems ⬆️55% over 2014

Reps ⬇️18% over 2014#txlege pic.twitter.com/AhV1sMY6VR — Brandon Rottinghaus (@bjrottinghaus) February 21, 2018

Political experts attribute much of Texas' increased voter turnout as a reaction to the election of President Donald Trump in 2016, as well as the state's eight open congressional seats.

"In general, there seems to be more energy, largely stemming from people's reactions to President Trump and a lot of Democrat-leaning groups trying to get people out and organized," said Robert Lowry, a political science professor at the University of Texas at Dallas. "It's maybe more Democrats than Republicans, but people who oppose him and don't like the results of the election and can't believe he won, [saying] 'We obviously can't vote against him this time but we can try to get more Democrats elected to respond to him.'"

He added that with Texas’s semi-open primaries, Republicans may vote in Democratic races and vice versa to help candidates who are a little more “acceptable,” or more moderate.

Lowry said the open congressional seats probably increased voter turnout because without an incumbent, both parties think they have a good chance to win.

In Congress, the eight Texans giving up their seats include three North Texas Republicans. Dallas Rep. Jeb Hensarling and Plano Rep. Sam Johnson are retiring, and Ennis Rep. Joe Barton is stepping down after a scandal in which sexual images he shared in an extramarital relationship were made public. Rep. Beto O’Rourke, D-El Paso, is leaving the House to challenge Sen. Ted Cruz for his seat.

Competitive congressional races also may push more Democrats to the polls, said Brandon Rottinghaus, a University of Houston political science professor.

“In counties where you have competitive races, Democrats are turning out other Democrats,” he said. “That’s the real difference between this midterm and 2014. There are a lot more Democrats running who are competitive in open seats, or in seats where they are likely to face a weakened incumbent.”

The difference since the last midterm election year, 2014, is significant. In heavily Republican Tarrant County, Democratic turnout was up 17 percent, while Republican turnout was down almost 10 percent. In Denton County, Democratic turnout was up 231 percent while Republican turnout was up 11 percent.

Collin County had 330 people vote in the Democratic primary on the first day of early voting in 2014 compared to 1,021 on Tuesday, a 209 percent increase. Republican turnout was up 37 percent.

According to the secretary of state's office, more than 15 million Texans were registered to vote as of November.

Texas Democratic Party Chairman Gilberto Hinojosa said in a news release that Texans’ votes are their voices and the best way to “stop Donald Trump, protect our communities and promote our progressive economic values.”

“Texas is a single-digit state and Texas Democrats are more competitive than ever before,” he said. “We have a historic number of Democratic candidates in virtually every level of office and the progressive values to move our state forward.”

Texas Republicans know that Democrats’ interest is high, but the numbers are from only one day of early voting, said James Dickey, Republican Party of Texas chairman.

"These numbers are similar to those from 2016, and Texans voted at that time by a significant margin to keep our state red,” Dickey said in a prepared statement. “I look forward to working with our Republican candidates and our grassroots volunteers to make certain that in 2018 we send a resounding message to Democrats across the nation that Texas is not a swing state."

Though early voting is a good way to measure enthusiasm, the first day’s results are not a reliable prediction for the rest of the primary, Rottinghaus said.

“It is hard to predict, because a lot can change,” he said.