Clinton and Obama, who waged an expensive Texas campaign that blanketed the state television ads, were locked in a dead heat with a quarter of precincts reporting.
While Obama enjoyed a strong showing in Dallas-Fort Worth and Houston, Clinton held the advantage in Bexar, Nueces and Hidalgo counties in South Texas. She had 77 percent of the vote in Webb County on the Texas-Mexico border, which includes Laredo, after early returns.
Clinton had counted on Latinos, who comprised 30 percent of the Texas Democratic primary vote, to help her in a state where she and former President Bill Clinton have political ties dating to the early 1970s. She was winning two-thirds of the Hispanic vote.
Exit polls showed that Obama won heavy margins among black voters, with a nearly 6-to-1 edge. Blacks accounted for 20 percent of the Democratic primary voters.
It appeared the overall voter turnout would set a state record and some polling places were still open more than two hours after the closing time to accommodate voters waiting in line.
"This is a historic election, and you are seeing historic turnout," said state Democratic Party spokesman Hector Nieto as party officials monitored delegate counts and caucusing from their Austin headquarters.
In the Republican race, Texas gave John McCain the delegates he needed to clinch the GOP nomination with a victory over Mike Huckabee, who dropped out of the presidential race. The former Arkansas governor had hoped Texas' social and religious conservatives would boost his struggling candidacy.
But with McCain so close to sealing the nomination, there was little interest in the Republican race in Texas compared with the close Democratic presidential contest, and large numbers of voters in conservative pockets of the state opted to vote in the Democratic primary.
In Collin County, one of the state's most Republican counties, Democratic turnout was five times its normal size, according to local media reports.
McCain had the backing of state GOP elected officials, including Gov. Rick Perry, who originally endorsed moderate Republican Rudy Giuliani and irritated some in his social conservative base with that decision. His later choice of McCain -- who's not a favorite of some hard-line conservatives -- didn't go over well either with social conservatives, but that irritation didn't help Huckabee as he'd hoped.
Texas will send 140 delegates to the national GOP convention, almost all awarded based on the primary vote.
Most voter interest was geared toward the closer Democratic presidential contest, where Obama was leading Clinton in early returns in urban areas like Houston, Dallas and Austin, while Clinton was leading overwhelmingly along the Texas-Mexico border and in other heavily Hispanic areas.
Texans didn't wait until Election Day to vote in large numbers. An estimated 2 million people showed up during two weeks of early voting -- about 60 percent of the overall number state officials expected to vote in both party primaries -- and they overwhelmingly cast ballots in Democratic races.
In a complex system, nicknamed the "Texas Two-step," Democrats have 193 pledged delegates to dole out based on primary vote results and a series of caucuses that began with precinct conventions after the polls closed Tuesday. Thirty-five others are unpledged delegates. Only voters who cast a ballot in the Democratic primary could take part in a caucus.
Texans didn't expect to figure prominently in the presidential nomination race after the Legislature declined last year to move up the state's primary to Feb. 5, leading to widespread speculation that the presidential nomination would be decided well before the race ever got to Texas.
University students Amy Hernandez, 20, and Monique Cano, 19, in Edinburg were elated Tuesday as they voted in their first primary and got to be part of the nomination decision. They had watched all the presidential debates and attended Clinton and Obama rallies. While Obama has seemed to be a big hit on college campuses across the country, they said they voted for Clinton.
"I honestly thought Obama was just full of talk," Hernandez said. "Hillary has been there in the White House. ... She'll be ready to get down to business."
Cano noted that Bill and Hillary Clinton have visited the Rio Grande Valley several times. "They know what's going on down here," she said.
Johnnie Johnson, a 25-year-old mortgage broker in San Antonio, voted for Obama. He's originally from Chicago and has long been a supporter of the Illinois senator, but said he's been particularly turned off by Clinton the last couple of debates.
"She didn't look presidential. She wasn't calm. She wasn't focused on issues," he said.
At times the presidential race overshadowed other races on the Texas ballot.
Four Democrats competed for their party's nomination for U.S. Senate. The winner is likely to face Republican Sen. John Cornyn, who had nominal primary opposition.
In congressional races, 10 Houston-area Republicans battled to become the one to face U.S. Rep. Nick Lampson in the fall general election, while two well-funded Republicans in the San Antonio area competed to take on Democratic Rep. Ciro Rodriguez.
Several Democratic Texas House members closely tied to Republican Speaker Tom Craddick tried to fight off primary challenges from opponents who took issue with their alliance with Craddick. The speaker, criticized from some legislators in both parties as too iron-fisted, took control of the House in 2003.
Republican primary voters approved three ballot resolutions urging that federal immigration laws be upheld, that voters be required to have a photo ID and that local spending be limited unless there's voter approval.
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