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Battleground Tracker: Republicans lead for Senate control; Iowa, Colorado tighten

Senate control tilts toward the Republicans as we head into the final days of campaigning, with the latest Battleground Tracker finding a number of tightening races. And with so many states in play, the GOP can take many paths to get there, though none of them looks easy, or certain.

The findings suggest it may now come down to two usually-blue states, Colorado - which has now tightened - and Iowa; along with a potential Republican pickup in Alaska. The possibility of runoffs in Louisiana and/or Georgia loom to complicate the mix. Either way it looks like the Senate majority could eventually be known late on election night with those western states, or else, if the Democrats manage to hold them, possibly in December and January when those southern battlegrounds would hold their runoffs.

Iowa - now a tie between Democrat Bruce Braley and Republican Joni Ernst - has voted for President Obama twice, but now looms as a blue state where the Republicans are in position to gain a seat. It was marginally leaning to Braley in the last Tracker round three weeks ago. And in Colorado, Democrat Mark Udall is clinging now to a one-point edge, well within the margin of error, as his lead has dwindled from three points in the last Tracker and four this summer.

Though few voters have shifted overall, Gardner shows some gains in positive perceptions: 22 percent say what they've seen or heard of him in recent weeks has made them think better of Gardner, while fewer 15 percent say that about Mark Udall.

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Republicans continue to hold edges for pickups in Alaska - Dan Sullivan up four on incumbent Mark Begich, though that has tightened a shade for Begich - and in Arkansas, where things appear to be moving in challenger Tom Cotton's direction; he is up five now from four earlier this month.

But as these races trend slightly Republican, Georgia has gotten tighter for Democrat Michelle Nunn. She's down three points now from six earlier this summer (and excluding "leaners," those who say they're only leaning to one candidate or another, Georgia is tied.) The bigger question may be whether the state heads to a runoff election if no one reaches 50 percent.

Both candidates have evenly balanced favorable/unfavorable ratings. But, as in Colorado, the findings suggest the campaigns are having some effect. Twenty-seven percent say what they've seen or heard about Nunn in recent weeks has made them think better of her, compared to 15 percent who say that of Perdue.

Meanwhile in Kansas, incumbent Republican Pat Roberts has bounced back: he was tied with independent Greg Orman in the last Tracker, which threw a curve into the Republicans' Senate math, but Roberts now shows a four point lead. Roberts' share of conservatives remains essentially unchanged but Orman's lead among self-described moderates has dropped a bit; the percent of moderates who are now unsure of their vote has edged up.

And in North Carolina, another southern state where Republicans are hoping for a pickup, Democratic Senator Kay Hagan holds and slightly expands her lead, up to three points now from two last time. Her campaign has continued to push local and state issues while Republicans, as everywhere, would like the race to be a referendum on the President.

The overall simulation, as last round, has the Republicans at 51 seats for a majority in the single most likely scenario, and now 52 seats is relatively more likely than 50, which speaks to the GOP's overall high likelihood of retaking the chamber.

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Nationwide, the undecided on the panel are mostly unsure who'll control the Senate after the election and tend to disapprove of how the President is handling his job. That suggests 2014 may now come down to a voter mobilization effort, if Democrats are to hang on to their majority, because the current demographic makeup of the likely electorate tends to favor the GOP.

The gender gap continues, favoring Democrats and helping them stay in the game. Women favor Democrats in every battleground state. Not much has changed in this regard since the last wave of data.

But the 65-and-over share of the likely electorate - which would be larger in 2014 than in 2012 - continues to favor Republicans by a comfortable margin in contested Senate races.

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How it works

Data in the fourth wave of the New York Times/CBS News Battleground Tracker are based on online interviews with 98,411 registered voters conducted October 16-23 in all 50 states and 435 Congressional districts across the United States.

YouGov, of Palo Alto, Ca., assembled the panel of registered voters, and conducted the selection, interviewing and tabulation. All 194,264 panelists who participated in one or more of the previous waves were recontacted. Of these, 87,864 were reinterviewed and an additional 10,547 panelists were added. All panelists have previously opted-in to a YouGov or other online survey panel. Reported tabulations are based on 86,323 respondents who said they would "definitely" or "probably vote" in the election to be held on November 4.

In each state and district, respondents were selected to match the demographics of registered voters. In competitive House districts and the 15 smallest states with Senate elections this year, YouGov interviewed disproportionate numbers of voters to increase the sample size for these races.

Respondents were matched and then weighted to demographics from the 2010 U.S. Census Congressional District Summary File (age, sex, race and Hispanic origin), the 2012 American Community Survey (education, marital status, employment status, home ownership and citizenship), and the Current Population Survey November 2012 Registration and Voting Supplement (voter registration and turnout). Past voting information is based on the 2012 National Election Pool Exit Poll and 2012 election returns.

To build the model, we have quite a bit of information about the people that are not in our sample and the races in those districts. From the Census, we know their demographics. From the 2012 election returns, we know the proportion who voted for each candidate in 2012 (or didn't vote). From the 2012 exit poll, we know the relationship between voter demographics and 2012 vote. And from out 2014 panel, we have data on how these variables relate to 2014 voting intentions.

We have combined these data into a statistical model that predicts 2014 vote on the basis of demographics and past vote. The model uses common patterns in the data to make estimates for people not interviewed. For example, if most of the 18-24 year old white female respondents in the sample who voted for Republican presidential nominee Mitt Romney in 2012 tell us that they intend to vote for the Republican congressional candidate in 2014, the model then predicts similar behavior for 18-24 year old white female voters in a district where our sample doesn't include any voters of this type. Where we have a few voters in a particular group, we average the model predictions with the sample, with the model estimates discounted as the sample size in that group increases. These techniques have been developed by statisticians and are commonly used for small area estimates by the Census.

The output of the statistical model is a range of estimates for each state and congressional district, reflecting the uncertainty surrounding the model's predictions. We have made 1,000,000 random draws from these estimates, producing thousands of different combinations of election outcomes. Some show Republicans winning both houses, others show Democrats maintaining control of the Senate, with countless variations of outcomes for particular races. In these simulations, we can calculate the proportion of times that, say, the Republicans gain a 51 or more seats in the Senate. This is the probability of Republican control of the Senate implied by the model.

The Battleground Tracker uses a different methodology from that of the regular CBS News Poll. The CBS News Poll is conducted using RDD sampling and telephone interviews.

The reported "margin of error" is an estimate, based upon a statistical model of the variability that would result theoretically from repeated applications of the same procedures. The practical difficulties of conducting any measure of public opinion may introduce other sources of error. More information about the methodology and data is available at the YouGov.com website.

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    Anthony Salvanto is CBS News elections director