Updated at 7:20 p.m.: Revised to include comment from Rep. Beto O'Rourke, after being revised earlier with Sen. Ted Cruz welcoming the announcement made three hours earlier, and aides insisting he wasn't booed when President Donald Trump endorsed him at the NRA convention.

WASHINGTON — President Donald Trump announced Friday that he'll come to Texas in October to hold a "major rally" for Sen. Ted Cruz in the biggest stadium in Texas.

The revelation that he would come to the rescue of a man he derided as "Lyin' Ted" during the 2016 presidential campaign came in a tweet, weeks after Cruz had said he would welcome Trump's help in his effort to fend off El Paso Rep. Beto O'Rourke — and after Cruz aides insisted the senator hadn't actually asked the president to stump for him.

Cruz waited three hours before publicly acknowledging Trump's plan, calling it "terrific!"

Trump is deeply divisive, even in conservative Texas, and Democrats insisted that his involvement in the race will benefit their side at least as much as Cruz.

"I'm picking the biggest stadium in Texas we can find. As you know, Ted has my complete and total Endorsement. His opponent is a disaster for Texas - weak on Second Amendment, Crime, Borders, Military, and Vets!" Trump tweeted.

At least one venue in Texas holds more than 180,000, so that's a Texas-sized promise.

The president has made his support for Cruz's reelection plain for months. But this was the first time he has attacked the challenger as he comes to the aide of an erstwhile rival who called the future president a "pathological liar" and worse at the height of the 2016 contest.

They've become close allies since Trump's election. Cruz has wrapped himself in the president's tax and immigration policies and lauded him for stocking federal courts with conservatives.

The high-level reinforcement reflects the precarious circumstances facing Cruz, Trump and Republicans in Texas and nationally. Texas hasn't elected a Democrat statewide since 1994, but O'Rourke has given the GOP reason for concern. If Democrats can wrest control of the House in November, impeachment becomes a real possibility.

Even forcing Republicans to spend heavily to defend the Cruz seat would be a major accomplishment, diverting resources from contests elsewhere.

Terrific! Texas will be glad to see you. Working together, we’ve won major victories for the people of Texas: historic tax cuts, repealing job-killing regulations, rebuilding the military & confirming strong constitutionalist judges. And as a result, the Texas economy is booming! https://t.co/BkeHpUo1pn — Ted Cruz (@tedcruz) August 31, 2018

Cruz needs Trump supporters, and a presidential embrace will juice them up. But Democrats and many independents plan to cast ballots in November to signal dismay with the president, so his appearance could also stir anti-Trump passions.

Matt Angle, a longtime Democratic strategist in Texas, called it "the equivalent of a Democratic rally in Texas" just before Election Day.

"Come on down! bwahahaha," tweeted the Texas Democratic Party's deputy executive director, Manny Garcia.

O'Rourke pounced on news of a Trump-Cruz rally to drum up donations, painting it as a sign that Republicans are worried.

"This is urgent. Donald Trump just sent his first tweet attacking Beto," his campaign wrote in an appeal to donors blasted out within minutes. "With each brand-new poll, Trump and Ted Cruz are getting more nervous. They're seeing their slim lead disappear altogether. And they're going to attack us again and again and again."

The Democrat, noting the rocky history between Cruz and Trump, said later on Friday that the president's visit to Texas was "an indication that we are in contention in this race."

In a briefing with reporters 10 days ago about Trump's fall campaign plans, top White House political aides declined to say whether he would inject himself into the Texas race. Since then, an Emerson College poll was released showing Cruz ahead by just one percentage point. Other polls have shown a tight race, with O'Rourke closing in.

Trump declared his unabashed support for Cruz in Dallas, at a National Rifle Association convention in May.

"Full endorsement for this man, Ted Cruz," as cheers, hoots and whistles mixed with boos in the hall. "Boy that was very rousing," Trump said.

Cruz aides insist there were no boos, only outcries of "Cruuuuuz."

The mixed response reflected lingering annoyance at Cruz for holding back on endorsing Trump when the GOP nomination fight ended. It had been a bitter campaign. Trump asserted at one point that Cruz's father had been involved in the John F. Kennedy assassination.

On Friday, Democrats gleefully dusted off some of the barbs Trump and Cruz traded during the 2016 primaries, such as Trump's assertion that his rival "has accomplished absolutely nothing" for the people of Texas.

Why would the people of Texas support Ted Cruz when he has accomplished absolutely nothing for them. He is another all talk, no action pol! — Donald J. Trump (@realDonaldTrump) February 28, 2016

Contenders for the biggest stadium in Texas include AT&T Stadium, home of the Dallas Cowboys; Kyle Field at Texas A&M; Darrell K. Royal-Texas Memorial Stadium at the University of Texas; NRG Stadium, home of the Houston Texans; the Cotton Bowl in Dallas; and Texas Motor Speedway in Fort Worth.

The speedway president wasted no time making his case, bragging that his facility also has the biggest TV screen in the world — a feature that likely would appeal to Trump, whose rallies are raucous events at which the president riffs, riles and entertains for an hour or more.

"So I'm waiting for a telephone call" from the president, tweeted Eddie Gossage.

Whether it's Cruz or O'Rourke who ends up taking a victory lap in November remains anyone's guess.

And @TXMotorSpeedwayalso has the biggest television screen in the world. So I'm waiting for a telephone call from @realDonaldTrump. https://t.co/c96t1PEPoq — Eddie Gossage (@eddiegossage) August 31, 2018

If they could fill any of those venues, the rally would likely be the biggest in Texas political history.

The speedway holds 181,655, though the permanent seating capacity is 128,655.

The Cowboys' stadium, known to fans as the Death Star, holds at least 100,000 people. The team hosted 105,121 fans in 2009 against the New York Giants. NRG Stadium in Houston, by comparison, tops out at around 80,000 people.

Kyle Field's official capacity is 102,733, though 110,633 showed up for a game in 2014. The Longhorns are only slightly behind the Aggies, attracting 102,315 fans to DKR two years ago for a game against Notre Dame. The little-used Cotton Bowl in Dallas seats more than 92,000.

There could be political considerations, too.

Cowboys owner Jerry Jones and his wife, Gene, do not appear to have donated money to Cruz's campaigns, according to campaign finance records. They also didn't contribute to Trump's presidential run, though a partnership linked to Jones gave $1 million to Trump's inauguration.

Houston Texans owner Bob McNair, meanwhile, donated $7,500 to Cruz's first Senate run and then gave $500,000 to a super PAC supporting Cruz's presidential bid. He also gave $449,000 to the Trump Victory joint fundraising committee, while making a $1 million contribution to Trump's inauguration.

Both owners have generally sided with Trump in the debate over NFL players protesting during the national anthem.

Some recent polls show a dead heat. Others show the incumbent with a small edge. But the race is only now starting to heat up and Cruz and allies are chipping away at O'Rourke's image of cheery bipartisanship.

The state GOP has hit the challenger over recent days by highlighting his days as a punk rocker, and two arrests two decades ago that did not result in convictions. Cruz has painted him as a radical leftist, and blasted him for defending NFL players who kneel during the national anthem to draw attention to racially tinged police shootings.

Texas voters are only slightly more fond of Trump than voters nationwide. Trump beat Democrat Hillary Clinton in Texas by 9 percentage points — the lowest margin of victory in years for a Republican presidential nominee.

But as Cruz has argued, the Senate contest may hinge on turnout and enthusiasm, and the senator will benefit from a Trump visit that could fire up his base.

As recently as Tuesday, the Cruz campaign was trying to tamp down news reports indicating that Cruz was seeking help from Trump, even though he'd said on Aug. 6 that he would "welcome" such support and had spoken with Trump about it.

"We have not placed a formal request for a rally or event but Senator Cruz and President Trump speak often about the political landscape in Texas and how the President can be helpful," Cruz campaign manager Jeff Roe said in a statement provided by the campaign on Tuesday.