AUSTIN — After unprecedented candidate spending and voter interest, Texas' midterm elections thunder to a close on Tuesday.

The 2018 results could be a harbinger of a national political shift and offer a whiff of change coming in Texas' beet-red voting patterns and conservative governance. Or not.

Some key races may be close, said Texas Christian University political scientist James Riddlesperger.

"Who has any idea how they're going to turn out," he said of the most competitive races. "We're going to be up late.”

Both major political parties have resorted to "fear tactics" to drive turnout by their most passionate followers, said Southern Methodist University professor of communication studies Rita Kirk.

"That would include [messages saying] everything from 'your Medicare is going to be ripped away' to 'the folks coming in the caravan are going to murder people and rape your children' and all of that," Kirk said. "So the fear tactics are really interesting but really hurt us in many ways as a body politic. ... They don't lead us to reasonable debate and reasonable conclusions."

TCU's Riddlesperger said he's most closely watching Democratic El Paso congressman Beto O'Rourke's bid to oust GOP U.S. Sen. Ted Cruz, state Attorney General Ken Paxton's race against Democrat Justin Nelson and a hand full of races for U.S. House, Texas Senate and Texas House — many of them in the Dallas-Fort Worth area.

Polls, which open at 7 a.m. and close at 7 p.m., stretch across two time zones — Central Standard Time in most of the state but also Mountain Standard Time in O'Rourke's hometown of El Paso.

In 12 days of early voting, which ended Saturday, more than 4.1 million Texans cast ballots in the 15 most populous counties.

That compared with nearly 4.5 million early votes in the 15 biggest counties in the 2016 presidential election. It more than doubled the number of early ballots cast early in those counties cast in the past two midterms, 2014 and 2010. In each cycle, just more than 1.7 million Texans voted early in the 15 most populous counties.

How many come out on Election Day is anyone's guess, said TCU's Riddlesperger.

"Does that [early voting] mark the fact that the voters were motivated, so they all went and voted early? Or is it just a precursor to another large turnout tomorrow," Riddlesperger said.

SMU's Kirk said she wonders how much of the increase in early-vote turnout is attributable to "shaming mailers" — direct mail pieces designed to embarrass a voter by listing his or her recent lack of participation in elections. Both sides have used them, she said.

"It makes people angry, but it also works," Kirk said.

On Monday, GOP numbers cruncher Derek Ryan issued a report that projects stout turnout on Election Day — which with the early vote would create a record-smashing total turnout for the midterm of 2018.

"I am leaning towards over 8 million people voting (Election Day + early combined)," Ryan, founder of Austin-based Ryan Data & Research, a Republican consulting firm, wrote in an email.

"That would be 52.2 percent turnout, which would be higher than any previous mid-term, but just short of presidential election percentages," he said.

Ryan said he based his projection on "how the top 15 counties voted during early voting, what their likely Election Day numbers will look like, and then assumes that these counties will make up 65 percent of all votes cast in the election."

The Texas Secretary of State’s office does not release early vote totals for less populated counties until late Tuesday, when all vote results are published.

Among the key races to watch:

Sen. Ted Cruz, R-Texas, left, greets the crowd during a campaign event Monday, Nov. 5, 2018, in Cypress, Texas. Cruz is being challenged by Democratic U.S. Representative Beto O'Rourke. (AP Photo/David J. Phillip) (David J. Phillip / AP)

U.S. Senate — The Cruz-O'Rourke contest is the night's marquee race.

"It is the most interesting election we've had in Texas in 24 years," said Riddlesperger, referring to Democrats' last statewide victories in 1994.

U.S. House — "There's a half dozen House elections I'll be watching, and they'll obviously include the three Republican-held congressional districts that Hillary Clinton won in 2016," the TCU professor said.

He was referring to the 32nd Congressional District in Dallas, where Rep. Pete Sessions is trying to hold off Collin Allred; the 7th in Houston, where Rep. John Culbertson faces Lizzie Pannill Fletcher; and the sprawling 23rd in southwest Texas, where Rep. Will Hurd is locked in a fight with newcomer Gina Ortiz Jones.

"I look for the Republican in Joe Barton's district to win, and suspect Kenny Marchant's going to get re-elected," Riddlesperger said, referring to two North Texas congressional seats. "But those districts are just edgy enough that something interesting could happen."

Other districts with competitive contests include an open-seat battle in a Central Texas-San Antonio district for retiring GOP Rep. Lamar Smith's seat; and Georgetown Republican Rep. John Carter's attempt to suppress surprisingly strong challenge from Democrat and military veteran Mary Jennings "MJ" Hegar.

Statewide — In the governor's race, GOP incumbent Greg Abbott has held a commanding lead in polls over former Dallas County Sheriff Lupe Valdez.

Rep. Beto O'Rourke (D-Texas), the Democratic challenger to Sen. Ted Cruz (R-Texas), at the House of Blues in Houston, Nov. 5, 2018. (Todd Heisler/The New York Times) (TODD HEISLER / NYT)

O'Rourke's strong performance for the Democrats, though, could depress the size of expected victories by Abbott and other Republican statewide officeholders, including Lt. Gov. Dan Patrick, Attorney General Ken Paxton, Comptroller Glenn Hegar, Agriculture Commissioner Sid Miller, Land Commissioner George P. Bush and Railroad Commission Chairwoman Christi Craddick.

Riddlesperger, though, said "I'll be keeping a little eye on the attorney general election."

He referred to the re-election bid of Paxton, of McKinney, who is running for re-election under the shadow of three felony indictments.

Paxton is the only top state leader who's released attack ads against his Democratic challenger. Opponent Justin Nelson has fired back, calling Paxton's accusations "laughably untrue" and saying the incumbent is "shaking in his boots."

On Monday, Paxton admitted the race may be competitive when he told conservative radio host Chad Hasty it "could be close."

Legislature — The GOP remains firmly in control, with lopsided majorities in each chamber.

Still, Tuesday presents an opportunity for Democrats to resume a project they began in the mid-2000s, before steady gains they posted in the Texas House were washed away in the Republican wave election of 2010, Riddlesperger said.

"The question is will the Democrats put themselves into a position where in two years, they could be in a position to be more competitive statewide," he said.

With moderate GOP Speaker Joe Straus of San Antonio retiring, "the subtext in the Texas House is are Democrats going to pick up enough seats to help engineer the election of another Joe Straus type?" Riddlesperger said. "Or are you going to end up with more of a Tom Craddick type," he said, referring to a conservative GOP speaker from Midland -- Christi Craddick's father. Straus toppled him in 2009.

In the Texas Senate, Riddlesperger and Matt Angle, a Democratic strategist, said former Burleson school board member Beverly Powell has mounted a respectable challenge to Colleyville GOP Sen. Konni Burton.

Don Huffines' run for re-election is also viewed as a possible opportunity for a Democratic pickup. Huffines, a Rand Paul Republican, is fighting against a challenge from Nathan Johnson, a moderate Democrat who has promised estranged GOP voters that he'll help pull their party back toward the middle. Senate District 16, wholly contained within Dallas County, went for Hillary Clinton in 2016.

Even if Huffines loses, however, Patrick is likely to retain a supermajority in the Senate because Democrats over the summer blew a special Senate election in San Antonio and southwest Texas.

Dallas County DA — Incumbent Republican Faith Johnson, an Abbott appointee, is trying to hold on to her job against Democrat John Creuzot.

In her nearly two years as Dallas County's top prosecutor, Johnson won praise when her office also successfully prosecuted a white police officer for the on-duty shooting of an unarmed black teenager.

Creuzot, a former judge known for criminal justice reform, is running largely on his judicial credentials, which include establishing one of the first drug courts in the state.

State government correspondent Lauren McGaughy in Austin and reporter Tasha Tsiaperas in Dallas contributed to this report.