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 GardnerSenate Republicans running away from Alabama abortion law Frustrated GOP senators want answers from Trump on Iran Graham: Trump officials not adequately briefing on Iran threat MORE (R) holds a slim lead over Sen. Mark UdallMark Emery UdallDenver Post editorial board says Gardner endorsement was 'mistake' Gardner gets latest Democratic challenge from former state senator Setting the record straight about No Labels 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 RobertsWomen's civil rights are not a state issue The Hill's 12:30 Report: Tough questions await Trump immigration plan Pat Robertson: Alabama 'has gone too far' with 'extreme' abortion law MORE (R), 45 to 44 percent. Orman was up 10 points earlier this month.

Arkansas:

Rep. Tom CottonThomas (Tom) Bryant CottonSenators offer bipartisan bill to help US firms remove Huawei equipment from networks GOP senator: Supreme Court abortion cases were 'wrongly decided as a constitutional matter' Senate confirms controversial 9th Circuit pick without blue slips 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 HaganNorth Carolina businessman will challenge Tillis in GOP primary Embattled senators fill coffers ahead of 2020 NC state senator meets with DSCC as Dems eye challenge to Tillis 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.”