LAS VEGAS (Reuters) - Outside political money is pouring into a race in Nevada to replace U.S. Democratic Senator Harry Reid, who is retiring and who told Reuters that the main reason his favored successor isn’t winning easily is cash from beyond the state’s borders. In a race that underscores the rise of outside money in U.S. campaign finance, as well as a key demographic challenge facing the Republican Party, Catherine Cortez Masto and Joe Heck are in a dead heat to replace the Senate’s No. 1 Democrat. Democrat Cortez Masto, 52, was a two-term Nevada attorney general. If she wins, she will be the first Latina elected to the U.S Senate. Republican Heck, 54, is a three-term House of Representatives member, physician and U.S. Army Reserve general. Polls show a tight race, with the two tied at 37 percent of likely voters in a mid-August Suffolk University survey.  The seat they are contesting is the only one in the Senate this year that Republicans have a truly good chance of flipping to their control from the Democrats. Doing so would shield Republicans’ Senate majority, which is under threat elsewhere. Saying in an interview that Heck’s campaign is fueled by outside money, Reid, a former boxer, lashed out at the Koch brothers, the U.S. Chamber of Commerce and Karl Rove. “The Chamber of Commerce is nothing more than a front anymore for the Koch brothers. Karl Rove, he doesn’t raise that money. He just goes to the Koch brothers. They’re all fronts for the Koch brothers,” said Reid, 76. “Catherine’s going to win. And if the Koch brothers weren’t involved, she’d win by 15 points,” Reid said. Charles and David Koch are wealthy industrialists who spend millions of dollars supporting libertarian-leaning candidates, mostly Republicans, through a network of groups, some operating under a law that lets them keep their donors’ identities secret. Reid, who now walks with a cane and often appears in dark sunglasses, has long warred with the Kochs. “We’ve certainly seen our fair share of criticism from Senator Reid. But we want to talk about the issues and he moves toward the politics of personal attacks,” said James Davis, spokesman for the Koch-aligned Freedom Partners Action Fund. The Supreme Court, in its 2010 Citizens United decision, opened the door to a tidal wave of political spending by corporations and labor unions on political causes and candidates, reshaping U.S. campaign finance.  More than 20 political groups, including two Koch-aligned organizations and the Reid-aligned Senate Majority PAC, have poured well over $20 million into the Nevada Senate race so far, according to the Center for Responsive Politics, which tracks campaign spending. Overall, the spending favors Heck over Cortez Masto by nearly 2-to-1. Freedom Partners and another Koch-aligned group, Concerned Veterans for America, have spent nearly $6 million, mostly on ads portraying Cortez Masto as captive to special interests.  Rove, formerly an adviser to Republican President George W. Bush, is now a Republican activist. He co-founded Republican Super PAC American Crossroads. Spokesman Ian Prior called Reid’s accusations about Rove and the Kochs “funny.” Prior said, “Look, if Harry Reid is looking for a bogeyman to star in his dark fantasy movie, he should just cast himself.” The U.S. Chamber of Commerce is the largest lobbying group for corporations in Washington and closely aligned with the Republican Party. On Reid’s allegations that it is a front for the Kochs, Chamber spokesman Blair Latoff Holmes said, “This is not true, but it’s certainly not the first time Senator Reid has made unsubstantiated allegations.” Nevada’s biggest city is Las Vegas and nearly three-quarters of the state’s population lives in or near the casino capital. Nevada reflects the United States’ growing racial diversity. In 1970, Nevadans were 87 percent white and 6 percent Latino; by 2010, that ratio was 54 percent white and 26 percent Latino. Heck, who is white, has managed to get elected to Congress from an ethnically diverse congressional district split evenly between Republicans and Democrats. But Democrats have a 72,000-vote registration advantage over Republicans in Nevada and Latinos helped elect President Barack Obama in 2008 and 2012. Republican presidential nominee Donald Trump has angered many Latinos with his vow to build a wall on the Mexican border and to deport undocumented aliens. Heck has endorsed Trump. While Trump’s popularity has crumbled in some other states, he has stayed within 2 percentage points of Democratic presidential nominee Hillary Clinton in Nevada. Cortez Masto has been keen to link Heck to Trump. She said in an interview that Heck has followed Trump’s example of “mud-slinging, dirty campaigning.” Heck spokesman Brian Baluta said Cortez Masto has distorted  Heck’s record. “She talks about Joe more than she talks about herself and it’s always in the most vile and negative way,” Baluta said. 
U.S. Senate's Reid, in last round, jabs outside money in Nevada succession fight