With the race for the Republican presidential nomination too close to call and U.S. Sen. Bernie Sanders of Vermont still waging an energetic campaign against Democratic frontrunner Hillary Clinton, election officials are bracing for exceptionally high turnout in this year's Texas primaries, where early voting begins today.

“I think Republican turnout is going to blow the doors off,” said Harris County Clerk Stan Stanart. “We’re planning on 100,000 more than we’ve ever done before.”

Both major parties are holding their primaries on March 1, and this year the Texas contests come unusually early in the presidential nomination process. Usually, far more states have primaries and caucuses scheduled before the Lone Star State, meaning that a prospective nominee is typically on the way toward wrapping things up by the time Texans weigh in, depressing turnout.

Presidential election years draw the most voters. Despite important federal, state and local races on the ballot during midterm elections, turnout typically suffers without a high-profile race for the White House at the top of the ballot. In elections of any kind, turnout tends to increase with the stakes: if candidates appear locked in a fierce battle, more people show up at the polls.

The last time Texans saw a knock-down-drag-out primary race for president was in March 2008, when Clinton and Barack Obama were still engaged in a heated competition for the Democratic Party's nod. That drove up primary turnout to a record 4.2 million, including almost 600,000 in Harris County.

This year, requests for mail-in ballots in Harris County had already hit a record as of late last week, Stanart said. He projected 55,000 voters would request one before the Feb. 19 deadline — up from the last high, 37,000, set four years ago.

All told, Stanart said he expected close to 400,000 people will cast primary ballots in the state’s largest county. Whereas Democrats dominated interest in 2008, Stanart predicted three-quarters of the turnout in Harris County will be on the Republican side, where native son Ted Cruz and New York billionaire Donald Trump are duking it out at the top of a shrinking list of GOP candidates.

Texas, which has 155 Republican delegates and 252 Democratic delegates, is the largest prize in the dozen or so states holding contests March 1. (The New York Times has a handy calendar with schedules of the two parties’ contests and the number of delegates at stake in each).

A complicated set of procedures determines how GOP delegates are allocated in Texas. Given how close the that race has been so far, the rules make it unlikely that any candidate can take all 155 delegates, enhancing the competitive aspect of the race.

Texas Secretary of State Carlos Cascos said the presence of native Texans on the ballot could further gin up enthusiasm here.

“You’ve got at least two candidates that have ties to Texas, that being Jeb Bush and Ted Cruz. That’s going to generate a lot of interest,” Cascos said. “The closer we get to March 1, (there will be) a lot more excitement. You’ll be able to see it.”

Assuming current trends hold, Republicans are on track to experience their most competitive race in Texas since 1976, when Ronald Reagan and Gerald Ford crisscrossed the delegate-rich state in a contentious primary battle. Reagan beat Ford in Texas, which helped lay the groundwork for his primary victory four years later, but Ford eventually took the nomination in a showdown at the party's national convention.

Jim Henson, director of the Texas Politics Project at the University of Texas at Austin, said the record high turnout in 2008 remains an “outlier” that will be hard to surpass. But he said the fierce competition in the GOP race will make 2016 one for the history books — and could approach the levels set eight years ago.

“There is every expectation that it’s going to be competitive here even though Cruz has favorite-son status,” Henson said. “Republican turnout is just going to be historic.”

Though experts predict a higher-than-average number of voters making their voices heard in the primary contests, it’s still a small slice of the electorate deciding on the party’s presidential candidates. In the record year of 2008, one-third of registered voters turned out to vote in one of the primaries, with Democrats outvoting Republicans by better than a two-to-one margin.

The 2008 performance far exceeded the turnout of registered primary voters in 2000 (16 percent), 2004 (12 percent) or 2012 (16 percent), figures show.

Cascos, the secretary of state, said he has been visiting as many high schools and college campuses around Texas he can fit into his schedule, hoping to ensure young people have the opportunity and knowledge to vote this year. His office is also educating people about the state’s voter ID law ahead of the first presidential election to be held under its provisions.

“We’re hoping that we do a good a job as we can do to inform, to educate, to provide whatever information is necessary to cast a ballot,” Cascos said.

Voters with questions about how to participate in the upcoming primaries can visit www.votetexas.gov, or call 1-800-252-VOTE.