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 GardnerHillicon Valley: Trump fires top federal cybersecurity official, GOP senators push back | Apple to pay 3 million to resolve fight over batteries | Los Angeles Police ban use of third-party facial recognition software Senate passes bill to secure internet-connected devices against cyber vulnerabilities Democrats vent to Schumer over Senate majority failure MORE (R) holds a slim lead over Sen. Mark UdallMark Emery UdallHickenlooper ousts Gardner in Colorado, handing Democrats vital pickup Live updates: Democrats fight to take control of the Senate The 10 Senate seats most likely to flip MORE (D), 46 to 45 percent. In September, the incumbent led 48 to 42 percent.

Iowa:

Joni Ernst leads Rep. Bruce BraleyBruce Lowell BraleyThe Memo: Trump attacks on Harris risk backfiring 2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP 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 RobertsTrump's controversial Fed nominee stalled after Senate setback Business groups scramble to forge ties amid race for House Agriculture chair Republicans hold on to competitive Kansas House seat MORE (R), 45 to 44 percent. Orman was up 10 points earlier this month.

Arkansas:

Rep. Tom CottonTom Bryant CottonO'Brien on 2024 talk: 'There's all kinds of speculation out there' Loeffler isolating after possible COVID-19 infection More conservatives break with Trump over election claims MORE (R) has a 2-point lead over Sen. Mark PryorMark Lunsford PryorCotton glides to reelection in Arkansas Live updates: Democrats fight to take control of the Senate Lobbying world 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 Hagan10 under-the-radar races to watch in November The Hill's Campaign Report: Democratic Unity Taskforce unveils party platform recommendations Democrats awash with cash in battle for Senate 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.”