Polls show Americans also consider Democrats more open than Republicans to reasonable compromise. So while the public is lukewarm on Democrats, it's extremely chilly toward Republicans. In the most recent NBC News-Wall Street Journal poll, favorable and unfavorable views about Democrats were about even. But Americans rated Republicans unfavorably by a double-digit margin.
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In several key Senate races, Democrats have something else going for them too: legacy candidates who are sons and daughters of familiar political figures in their states.
Memories of his father Nick Begich, killed in a plane crash while holding a House seat, could help Democratic incumbent Sen. Mark Begich hold his seat in Alaska.
Memories of her father Moon Landrieu, a former mayor and Cabinet member, could help Sen. Mary Landrieu hold her seat in Louisiana.
And memories of her father Sam Nunn, a retired senator, could help Michelle Nunn win a seat in Georgia.
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All three Democratic candidates have featured their fathers in their advertising. The father of another vulnerable Democrat, Sen. Mark Pryor of Arkansas, served as both governor and senator there.
All four are red states that Obama lost in both 2008 and 2012. For his party to keep control of the Senate for the last two years of his presidency, one or more of them needs to win.
—By CNBC's John Harwood.