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 GardnerGOP senators unveil bill to expand 'opportunity zone' reporting requirements Overnight Health Care: House to vote next week on drug prices bill | Conway says Trump trying to find 'balance' on youth vaping | US spent trillion on hospitals in 2018 Giffords, Demand Justice to pressure GOP senators to reject Trump judicial pick MORE (R) holds a slim lead over Sen. Mark UdallMark Emery UdallDemocrats will win back the Senate majority in 2020, all thanks to President Trump Poll: Trump trails three Democrats by 10 points in Colorado The Hill's Morning Report — Trump and the new Israel-'squad' controversy 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 RobertsThe Hill's Morning Report - Intel panel readies to hand off impeachment baton The job no GOP senator wants: 'I'd rather have a root canal' The Hill's Morning Report — House set for Phase 3 of impeachment push MORE (R), 45 to 44 percent. Orman was up 10 points earlier this month.

Arkansas:

Rep. Tom CottonThomas (Tom) Bryant CottonTikTok's leader to meet with lawmakers next week Hillicon Valley: FTC rules Cambridge Analytica engaged in 'deceptive practices' | NATO researchers warn social media failing to remove fake accounts | Sanders calls for breaking up Comcast, Verizon Bipartisan senators call on FERC to protect against Huawei threats MORE (R) has a 2-point lead over Sen. Mark PryorMark Lunsford PryorTom Cotton's only Democratic rival quits race in Arkansas Medicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation 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 HaganGOP braces for Democratic spending onslaught in battle for Senate Democrats will win back the Senate majority in 2020, all thanks to President Trump The Hill's Morning Report - Presented by Better Medicare Alliance - Dems shift strategy on impeachment vote 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.”