WASHINGTON — The party that runs the Senate next year may be decided by how well President Barack Obama and Mitt Romney do in toss-up states like Nevada, Ohio, Virginia and Wisconsin, where ballots feature parallel Senate races about as tight as the presidential contest.

The mammoth campaign organizations built by Obama and Romney, his Republican challenger, are focusing their voter registration and turnout efforts in those four states and a handful of other presidential battlegrounds. Congressional candidates there are latching onto the help that can come from the larger, better-funded presidential campaigns.

In Nevada, Democratic Rep. Shelley Berkley is hoping to buttress her challenge to Republican Sen. Dean Heller with the Obama campaign’s efforts to register Hispanic voters. In Virginia, the GOP has operated 29 offices across the state combining the operations of Romney, Senate candidate George Allen and House candidates.

Democrats control the Senate 53-47, including two independents who vote with them. Of the 33 seats up for grabs on Election Day, a dozen are considered competitive, largely in the West and Midwest. Republicans need a net pickup of four seats to take control if Obama is re-elected, three if Romney wins.

Both sides are measuring the impact of the presidential race at a time when spending on congressional races — especially by outside groups — is mushrooming.

In the House, Democrats have been hoping that a strong Election Day performance by Obama could lift their candidates, especially in states he is expected to win easily like New York, Illinois and California. They may make some gains but seem unlikely to pick up 25 seats they need to wrest House control from the GOP. Only about 60 seats are considered competitive in the 435-member House.

“There’s no question we pick up seats in direct correlation to the president’s coattails,” said Rep. Steve Israel, D-N.Y., who heads the House Democratic campaign organization.

Rep. Greg Walden, R-Ore., deputy chairman of the House GOP’s political operation, concedes that a strong Obama showing would likeliest strengthen the Democratic vote in urban areas, where Republicans have few seats anyway. Republicans hope to limit Democratic pickups by winning seats of their own in North Carolina, Arkansas, Indiana and even Massachusetts.

“It helps us if Romney is doing better,” especially in rural and suburban areas where many GOP lawmakers come from, Walden said. “And Gov. Romney is doing much better” than he was earlier in the campaign, said Walden.

In Ohio, Republican challenger Josh Mandel is hoping for a late surge by Romney that might also lift him past Democratic Sen. Sherrod Brown. Polls show Brown, who has a liberal pro-labor voting record, consistently leading Mandel and doing better in the state than Obama, whose advantage in the pivotal state has narrowed.

On Friday, the U.S. Chamber of Commerce reported spending $2.3 million for TV and radio ads to help Mandel, making Brown one of the business lobby’s top targets.

“If Romney could keep it close, Mandel’s going to be in the Senate,” said Scott Reed, a top political strategist for the chamber.

In a debate last week, Mandel hammered his rival for supporting Obama’s health care overhaul and for driving up the national debt with efforts such as the federal auto rescue. Brown made no apologies, ticking off benefits he said each law brought to average Ohioans.  

“I’m proud of that because now more than a million Ohio seniors now get free checkups,” he said of his support for the health care law.  

In wide-open Virginia, the presidential race’s impact on the Senate contest may be tempered by the near universal name recognition of the two Senate rivals. Republican George Allen and Democrat Tim Kaine are former recent governors, and the $20 million the two have combined to spend so far makes their contest one of the nation’s most expensive, according to the nonpartisan Center for Responsive Politics.

“Headwinds will largely come from the presidential, but because name identification is so high for both Governor Allen and Governor Kaine, these races will be nearly mirroring each other,” said Pete Snyder, chairman of the GOP’s coordinated presidential and congressional campaigns in the state.

Ticket-splitting is as much as factor in some Senate races as coattails. Democratic Sen. Joe Manchin is heading toward re-election in West Virginia and there are tight Senate races in Arizona, Indiana and North Dakota, though Romney seems certain to win all four states.

Obama is sure to win Connecticut, yet professional wrestling magnate Linda McMahon, a Republican, is running a well-financed and strong race against Democratic Rep. Chris Murphy for the seat being vacated by independent Sen. Joe Lieberman.

The presidential contest is also providing plenty of fodder for congressional campaigns — and not just in the frequent attacks that Republicans make on Obama and Democrats launch against Romney.

In a play for moderate Virginia voters, Kaine uses one TV spot to position himself between Allen and Obama. The ad states Obama’s preference to block a renewal of decade-old tax cuts on income exceeding $250,000, and Allen’s insistence — shared by most Republicans — on extending the reductions for all.

“There’s a middle ground. Let the tax cuts expire for those earning over $500,000,” Kaine tells the camera, calling it “the fiscally responsible thing to do.”

In an Indiana ad, Senate Democratic hopeful Rep. Joe Donnelly attacks his GOP rival, tea party favorite Richard Mourdock, using presidential debate footage of Romney saying he got little done as Massachusetts governor “by saying it’s my way or the highway.” Mourdock “is all about my way or the highway,” the ad says.

And in a Wisconsin Senate debate last week, GOP candidate Tommy Thompson distanced himself from the House-passed budget written by Rep. Paul Ryan, the Republican vice presidential candidate. His rival, Democratic Rep. Tammy Baldwin, repeatedly linked him to the blueprint that she argued would raise taxes on the middle class.

“You’re absolutely wrong,” Thompson told Baldwin. “I have a plan completely different from Paul Ryan.”

Meanwhile, spending has continued to accelerate on congressional races by both political parties and outside groups, including Crossroads GPS backed by former White House GOP strategist Karl Rove and unions such as the SEIU.

According to the nonpartisan Sunlight Foundation, which monitors campaign spending, outside groups have spent $161 million since June 1 on House races and $156 million on Senate races, with Republicans benefiting from a modest advantage.

But that spending has accelerated in recent days, especially on the GOP side, the foundation found.

In the week ending last Friday, groups have spent $13 million to help House Democratic candidates and $22 million for House Republicans. Senate Democratic hopefuls have benefited from $6 million, Senate Republicans from $10 million.

In one instance, an obscure conservative group, the Government Integrity Fund Action Network, spent $1.1 million for an ad attacking Democrat Elizabeth Esty, who is seeking an open House seat in Connecticut. That is a huge expenditure for a House race.

The Senate Democratic campaign committee spent $1.6 million to advertise against Allen in Virginia.