- A CNN/ORC poll finds Republican Cory Gardner is leading Democratic Sen. Mark Udall, 50-46.
- Gardner's lead is built on huge advantages in Denver's suburbs and Colorado's rural areas.
- Udall is one of a handful of Democratic incumbents that Republicans have targeted this year.
Republican Cory Gardner has opened up a four-point lead in a Colorado Senate race that's key in determining whether Democrats can hold onto their majority, a new CNN/ORC poll shows.
Gardner held a 50 percent to 46 percent edge on first-term Democratic Sen. Mark Udall in the survey of 665 likely voters, conducted Oct. 9-13. The poll's margin of error is plus or minus 4 percentage points.
Republicans have targeted Udall and several other Democratic incumbents in their effort to win at least six seats this fall -- enough to gain a Senate majority for President Barack Obama's final two years in office. Traditional Democratic advantages among women and urban voters aren't enough to overcome strong headwinds, said CNN Polling Director Keating Holland.
"Udall is getting clobbered in the all-important suburbs, as well as in rural areas, and his lead in Colorado cities is not enough to overcome that," Holland said.
"There is a gender gap, but in this case it appears to be working in favor of the Republican. Udall has a nine-point advantage among women, but that is dwarfed by the 20-point lead Gardner has among men," he said.
Hurting Udall is an underwater favorability rating of 45 percent -- with 51 percent of those surveyed saying they have an unfavorable opinion of the Democrat.
Gardner, meanwhile, is viewed favorably by 50 percent of likely voters, with just 42 percent saying their opinion of him is unfavorable.
Colorado's governor's race is even closer. Incumbent Democratic Gov. John Hickenlooper is backed by 49 percent of likely voters, while Republican challenger Bob Beauprez has 48 percent support, the CNN/ORC poll found.
The Democratic incumbents are both outperforming Obama, who twice won the state's nine electoral votes but now has an approval rating of just 39 percent in Colorado, while 59 percent of likely voters say they disapprove of the job he's doing.