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.