LULU GARCIA-NAVARRO, HOST:
Hold on tight. Tuesday starts a two-month run of Senate primaries in a dozen states. As NPR's Peter Overby reports, it's going to be expensive for both Democrats and Republicans.
PETER OVERBY, BYLINE: It's a grand time to be a Democratic Senate candidate. Democrats have already reached 80 percent of their final Senate fundraising total for 2016. Republicans are lagging at 60 percent. That's according to the nonpartisan Center for Responsive Politics. Republican strategist Ron Bonjean said Democrats are doing a great job bringing in the cash.
RON BONJEAN: There's absolutely no question about it. At the same time, outside groups that are loyal to Republicans are also raising money hands over fists.
OVERBY: For Republicans, the not-so-secret weapon is the large number of conservative outside groups, superPACs and nonprofit organizations backed by wealthy, often anonymous donors. But Bonjean said the GOP candidates shouldn't count on them.
BONJEAN: Republican candidates need to step up and raise as much money as possible and not get into a false sense of security that these groups are going to be there to save the day.
OVERBY: Outside groups have flourished since 2010. That was when the Supreme Court's Citizens United ruling swung the door open for unlimited giving.
JENNIFER VICTOR: It's that money - it's that unregulated, independent expenditure money that we see an uptick every cycle since that law changed.
OVERBY: Jennifer Victor is a political scientist at George Mason University in Northern Virginia.
VICTOR: There's a lot of structural factors that, to some extent, are advantaging Democrats right now. And yet we still are seeing this uptick in the outside spending.
OVERBY: One race that demonstrates this is in Nevada. Dean Heller is the only Republican senator running for re-election in a state where Hillary Clinton beat Donald Trump. Heller and his challenger, Democratic Congresswoman Jacky Rosen, have raised almost $6 million apiece. Rosen is way ahead among small donors, those who give $200 or less. But Heller will likely get more help from outside groups where conservatives have more muscle. Megan Jones is a consultant to some of the liberal groups. She says the emphasis is on voter contacts, not TV ads.
MEGAN JONES: There's a lot of national investment. But that national investment is being used to create infrastructure and leaders on the ground.
OVERBY: The nation's two top donors from 2016 are likely to play prominent roles. Liberal Tom Steyer already has his organization working in Nevada. And conservative Sheldon Adelson of Las Vegas hasn't invested in Heller yet, but he is backing the GOP candidate for governor, Adam Laxalt. Again, Megan Jones.
JONES: He is very much invested in ensuring that Laxalt wins the governor's seat. I think that will definitely spill over into the Senate race.
OVERBY: A race that could end up deciding who controls the Senate next year - Peter Overby, NPR News, Washington.
(SOUNDBITE OF STEVE HAUSCHILDT'S "POLYHYMNIA") Transcript provided by NPR, Copyright NPR.