New results from NBC News/Marist polls show close races in midterm contests that will decide control of the U.S. Senate.

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The polls found momentum in most of the races has swung to the Republicans, however.

Colorado:

Rep. Cory GardnerCory Scott Gardner The 13 Republicans needed to pass gun-control legislation Bolton returns to political group after exiting administration The Hill's Morning Report — Trump's hurricane forecast controversy won't go away MORE (R) holds a slim lead over Sen. Mark UdallMark Emery UdallPoll: Trump trails three Democrats by 10 points in Colorado The Hill's Morning Report — Trump and the new Israel-'squad' controversy Colorado candidates vying to take on Gardner warn Hickenlooper they won't back down MORE (D), 46 to 45 percent. In September, the incumbent led 48 to 42 percent.

Iowa:

Joni Ernst leads Rep. Bruce BraleyBruce Lowell Braley2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward MORE (D), 49 to 47 percent. Her lead was 2 percentage points, 46 to 44 percent, earlier this month.

Kansas:

Greg Orman (I) holds a slight lead over Sen. Pat RobertsCharles (Pat) Patrick RobertsHere are the lawmakers who aren't seeking reelection in 2020 Hillicon Valley: Lawmakers ramp up Silicon Valley antitrust probe | Treasury sanctions North Korean cyber groups | Thiel to host Kobach fundraiser Ann Coulter, Peter Thiel slated to host fundraiser for Kobach's Senate bid: report MORE (R), 45 to 44 percent. Orman was up 10 points earlier this month.

Arkansas:

Rep. Tom CottonThomas (Tom) Bryant Cotton2020 Democrats raise alarm about China's intellectual property theft Bolton returns to political group after exiting administration Meadows, Cotton introduce bill to prevent district judges from blocking federal policy changes MORE (R) has a 2-point lead over Sen. Mark PryorMark Lunsford PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D), 45 to 43 percent. Cotton held a 5-point lead last month.

North Carolina:

The new NBC News/Marist poll shows Sen. Kay HaganKay Ruthven HaganTillis trails Democratic challenger by 7 points in North Carolina poll North Carolina businessman will challenge Tillis in GOP primary Embattled senators fill coffers ahead of 2020 MORE (D) deadlocked with Republican opponent Thom Tillis, with each candidate taking 43 percent support. Libertarian Sean Haugh takes 7 percent. Hagan had a 4-point lead earlier this month.

All the races are within the polls’ margins of error.

Pollster Barbara Carvalho of Marist College’s Institute for Public Opinion said, unlike 2012, “Democrats are now struggling to protect their firewall in Iowa, North Carolina and Colorado.”