Candidates running for federal office in Nevada are raising and spending millions upon millions of dollars in what has been the most expensive midterm election in U.S. history.

Sen. Dean Heller, R-Nev., left, and Rep. Jacky Rosen, D-Nev., shake hands before a U.S. Senate debate, Friday, Oct. 19, 2018, in Las Vegas. (AP Photo/John Locher, Pool)

CARSON CITY — Candidates running for federal office in Nevada are raising and spending millions upon millions of dollars in what has been the most expensive midterm election in U.S. history.

More than $4.7 billion has been spent this election cycle, according to the Center for Responsive Politics, and the spending is projected to be 35 percent higher than the money flow seen in 2014.

Nearly a quarter of the 2018 spending has come from outside groups trying to influence voters.

Outside groups on both sides of the political aisle are likely to have an outsized impact on the state’s most competitive races, with a combined $77 million from outside Nevada being pumped into the state’s races for U.S. Senate and the open Congressional Districts 3 and 4.

Those groups have seen their level of influence grow in recent elections, thanks in large part to the Citizens United Supreme Court decision from 2010, which opened the door to effectively unlimited spending by organizations, super PACs and political nonprofits — often referred to as “dark money” groups because they do not have to disclose their donors.

The U.S. Senate race in Nevada between incumbent Republican Sen. Dean Heller and Democratic challenger Rep. Jacky Rosen is a prime example.

The last time Heller was up for election, in 2012, outside groups spent a total of $25.5 million.

Jump to 2018, and outside groups have dumped some $58 million into the race as of Tuesday, according to the Center for Responsive Politics.

Here’s a look at how much money has been spent through Tuesday on Nevada’s Senate and two competitive congressional races — and where it’s coming from:

U.S. Senate — $58 million

Rosen has outspent Heller $20.5 million to $12.2 million in what is one of the most closely watched races in the country and one that could determine which party controls the Senate during the second half of President Donald Trump’s first term.

More than $30.5 million has been spent by outside groups opposing Heller, compared with $18.6 million by groups opposing Rosen. Groups supporting Heller spent a smaller $3.9 million, while groups supporting Rosen spent just over $4.6 million.

The biggest outside spender in the race has been the Senate Leadership Fund, the super PAC tied to Senate Majority Leader Mitch McConnell that is trying to protect the Republican hold on the Senate, which has spent $14.1 million on the race.

The Senate Majority PAC, the Democratic super PAC tied to Senate Minority Leader Chuck Schumer, comes in at No. 2 with $12.3 million in spending.

But those major spenders aren’t the only ones trying to swing the vote. Behind those two leadership PACs, nine other political groups have spent in excess of $1 million trying, including the Democratic Senatorial Campaign Committee ($6.3 million) and National Republican Senatorial Committee ($4.9 million).

CD-3 — $9.3 million

Democratic nominee Susie Lee has more than doubled the amount spent by her Republican opponent Danny Tarkanian. But outside groups — including one tied to Trump — are hoping to to boost Tarkanian to his first victory in his sixth run for office.

Republican group shave spent about $4.7 million attacking Lee, while Democratic groups have spent a combined $4.4 million going after Tarkanian.

The Congressional Leadership Fund, the group hoping to protect the Republican majority in the House, and the Democratic Congressional Campaign Committee have each dropped just over $3 million on the race.

And America First Action, the pro-Trump super PAC, has put in $1.6 million to fund ads attacking Lee, while the Nancy Pelosi-tied House Majority PAC has spent $1 million to attack Tarkanian.

CD-4 — $9.9 million

The race between Democrat Steven Horsford and Republican Cresent Hardy has seen outside groups outspending the candidates about five-to-one.

Of the nearly $10 million in outside spending, more than two-thirds of that have gone to boost Horsford’s chances.

Nearly all of the other $2.9 million spent by outside groups in the race to boost Hardy’s shot in the race have come from the National Republican Congressional Committee.

Horsford, meanwhile, is benefiting from groups like Independence USA PAC ($2.8 million) — the super PAC tied to former New York City Mayor Michael Bloomberg, the DCCC ($2.5 million) and the House Majority PAC ($1.1 million).

The Review-Journal is owned by the family of Las Vegas Sands Corp. Chairman and CEO Sheldon Adelson. The Adelson family this year has combined to donate $30 million to the Congressional Leadership Fund, $25 million to the Senate Leadership Fund, $10 million to America First Action, $305,100 to the NRCC, $67,800 to the NRSC and $10,800 each to the campaigns of Heller and Hardy, according to Federal Election Commission records.

Contact Capital Bureau Chief Colton Lochhead at clochhead@reviewjournal.com or 775-461-3820. Follow @ColtonLochhead on Twitter.