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By Perry Bacon Jr.

Republican candidates lead in key Senate races in Arizona, Georgia, Nevada and New Hampshire, according to new NBC/Wall Street Journal/Marist polls.

These results put the GOP on the path to retain control of the Senate, even if Hillary Clinton wins the presidential election. And they are another illustration of the weakness of Donald Trump, who is either losing or barely winning in states where other Republican candidates are doing much better.

In New Hampshire, incumbent Republican Kelly Ayotte is ahead 52 to 44 percent over Democratic challenger Maggie Hassan. In Nevada, Republican Joe Heck leads Democrat Catherine Cortez Masto 47 to 45 percent. Arizona's John McCain has a 57-38 lead over Ann Kirkpatrick and Georgia Republican Johnny Isakson has a 53-38 advantage over Jim Barksdale.

Trump is effectively tied in all four states with Clinton.

In part, these Republican candidates, three of whom are incumbents, benefit from being better well-known than their Democratic challengers. And other polls have shown Hassan either leading or much closer to Ayotte in New Hampshire.

But if these numbers hold, they will have major implications. Democrats currently hold 46 seats in the Senate, and they are favored to win races in Illinois, Ohio, Pennsylvania, and Wisconsin. If Democrats won 50 seats and Clinton were elected president, Tim Kaine would be the tie-breaking 51st vote as vice president.

But Nevada's Heck, if he won, would replace Harry Reid, the outgoing Nevada Democratic senator. His victory would give Republicans 51 seats and a majority.

Whichever party controls the Senate will have huge influence next year, with the potential to bloc or approve Supreme Court and cabinet nominees in particular. if Clinton won but Republicans kept control of both houses of Congress, she would face the same challenges Obama has, essentially unable to get legislation approved on Capitol Hill.