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 GardnerThe Hill's Campaign Report: Democratic field begins to shrink ahead of critical stretch The Hill's 12:30 Report: Democratic field narrows with Inslee exit Inslee seeking third term as governor after ending presidential bid 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 BraleyOPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer 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 RobertsThe Hill's Campaign Report: Democratic field begins to shrink ahead of critical stretch Kobach says he's more prepared for 'propaganda' in Senate campaign Pompeo: Senate run 'off the table' MORE (R), 45 to 44 percent. Orman was up 10 points earlier this month.

Arkansas:

Rep. Tom CottonThomas (Tom) Bryant CottonGOP senator says he suggested Greenland purchase to Trump, met with Danish ambassador It's time to empower military families with education freedom Cotton warns China: Crackdown on Hong Kong would be 'grave miscalculation' 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.”