HARRISBURG, Pa. -- Billions of dollars in advertising are raining down on voters across the Rust Belt, Rocky Mountains and American southwest as the two major political parties portray their opponent's candidates as extreme in a struggle for control of the U.S. Senate.

In three races alone — Ohio, Pennsylvania and Montana — more than $1 billion is projected to be spent by Nov. 5.

The race in Ohio could break the spending record for Senate races. The race in Montana will go down as the most expensive Senate race ever on a per-vote basis. And, late in the game, Democrats are sending millions more dollars to Texas, a GOP stronghold where the party has new hopes of knocking off two-term conservative stalwart Sen. Ted Cruz, an upset that could help them protect their majority.

Republicans need to pick up two seats to capture a surefire majority, and one of those — West Virginia — is all but in the bag for the GOP.

Other races are more volatile and less predictable.

For Democrats, the brutal math of this year's election cycle is forcing them to defend eight seats in tough states. Losses by established incumbents could amount to an extinction-level event for Democrats who represent reliably Republican states.

The election also will test the down-ballot strength of both parties in Wisconsin, Michigan and Pennsylvania, the premier presidential battleground states known as the Blue Wall for their relatively reliable Democratic voting history. Wins there by Republicans would dramatically alter the Senate playing field.

All told, data from political ad tracking firm AdImpact projects that more than $2.5 billion will be spent on advertising in Senate races in this two-year campaign cycle, slightly more than the 2022 total.

That includes a half-billion dollars in Ohio alone, another $340 million in Pennsylvania and $280 million in Montana, population 1.1 million, or less than one-tenth of the population of either Ohio or Pennsylvania. The most expensive Senate race ever was Democrat John Ossoff 's victory in a Georgia contest that went to a runoff in 2021 and decided Senate control, according to data from the campaign finance-tracking organization Open Secrets.

Generally, campaign strategists say Republican presidential nominee Donald Trump is polling ahead of his party's Senate candidates in Senate battleground states, while Democratic candidates in those states are polling ahead of their presidential nominee, Kamala Harris.

That means there is a slice of voters who could vote for Trump but not back Republicans in Senate races — or who could split their tickets with Democratic Senate candidates.

Such splits have been rare. In Maine, in 2020 voters backed Democrat Joe Biden for president and re-elected Republican Sen. Susan Collins, for instance.

Republican strategists said they expect the party's major super PACs to spend until election day in seven states where Democrats are defending Senate seats: Michigan, Montana, Ohio, Pennsylvania and Wisconsin, where polls show competitive races, but also Nevada and Arizona, where Republicans are encouraged by strong early voting numbers.

Republicans are most confident about flipping the seat in deep-red Montana, where Republican Tim Sheehy is challenging third-term Democratic Sen. Jon Tester. They are also optimistic about reliably red Ohio, where Republican Bernie Moreno is challenging third-term Democratic Sen. Sherrod Brown.

Torunn Sinclair, a spokesperson for a pair of Republican-aligned super PACs, said one — American Crossroads — is pulling $2.8 million out of Montana, while the pair are plunging several million more into Pennsylvania.

There, Republican David McCormick is trying to knock off three-term Democratic Sen. Bob Casey in a presidential battleground undercard that both sides say is close.

McCormick, a former CEO of the world's largest hedge fund, has hammered the message in two debates that Casey is a “sure thing” to back the Biden-Harris administration's agenda.

In recent days, Casey began running an ad in conservative areas that touts his “greedflation” legislation to pursue price-gouging. The ad says “Casey bucked Biden to protect fracking" and “sided with Trump” on trade and tariffs.

Republicans say Casey’s ad showing Trump is similar to a TV ad that Sen. Tammy Baldwin of Wisconsin is airing and speaks to both Democrats’ need to protect themselves against Harris’ vulnerability in their states.

“They're hoping to peel off enough Trump voters to win,” Sinclair said.

Still, Casey ran a similar ad in 2018's midterm election when he won easily — even though that ad didn’t mention Trump — while Casey's campaign notes that he has long split with Democrats by opposing free trade agreements and supporting fossil fuel-power projects.

Democrats, conversely, say they are forcing competitive contests late in the campaign in two red states, Texas and Nebraska. Ousting incumbent Republicans from one or both of those seats could help Democrats to at least a 50-50 split in the Senate should Democrats lose in Montana or Ohio.

In Texas, U.S. Rep. Colin Allred, a former professional football player, has proven adept at raising small-dollar donations in his challenge to incumbent Republican Sen. Ted Cruz. Allred has outraised every Senate candidate nationally, except Tester and Brown.

The ad spending advantage for Allred has been 3-to-2, according to AdImpact, with the Democratic-aligned Senate Majority PAC touting a new seven-figure digital ad buy and a separate $5 million TV ad buy attacking Cruz on a key issue for Democrats, abortion rights.

On top of that, Democrats hope Harris' rally in Houston on Friday with Allred and Beyoncé can help Allred by boosting Black voter turnout.

In Nebraska, independent Dan Osborn — a tattooed former labor leader who supports abortion rights — appears to have consolidated Democratic and independent voters while making some inroads with Republicans, Democratic strategists say.

While Osborn is running as an independent and hasn't said which party he'd caucus with, he's getting support from a liberal super PAC that has helped him amass a significant spending advantage over Republican Sen. Deb Fischer.

In both states, Republicans acknowledge that they've had to spend money unexpectedly to shore up their incumbents' prospects, but they also say they expect to win comfortably.

In Ohio, Brown has tried to personalize his appeal by appearing in most of his own ads and speaking directly into the camera.

“I'm Sherrod Brown and I have a question," Brown says, looking into the camera and leaning his elbow on what might be a wood-working shop table. “Have you ever heard Bernie Moreno talk about what he's going to do for Ohio?”

Brown also makes a personal appeal to potential swing voters, saying he has spent his career fighting for workers and veterans and working with law enforcement and “presidents of both parties to do what's best for our state.”

Elsewhere, strategists expect first-term Florida Sen. Rick Scott will fend off a challenge from Democrat Debbie Mucarsel-Powell and that Democrat Angela Alsobrooks in deep-blue Maryland will beat former Gov. Larry Hogan to fill a seat being vacated by Democratic Sen. Ben Cardin.

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Associated Press reporter Julie Carr Smyth in Columbus, Ohio, contributed to this report. Follow Marc Levy at twitter.com/timelywriter.