Right now, the Real Clear Politics averages of all polls that are covering the Senatorial elections have the Democrats and the Republicans holding 45 seats each; leaving 10 seats as toss ups and too close to call. This, then, means that whatever party is able to win 6 of those 10 races will have majority control of the Senate come January.
But, the problem with almost all of these polls is that they don't reflect the historical voter demographics of midterm elections which tend to give Republicans a natural advantage.
First of all, the midterm voters tend to be older; and older voters more often vote Republican. Also, a large part of the Democrat's base, Blacks, tend not to vote in the midterms. This also proves true for women, who historically favor Democrats. On the other hand, non-black males show up and tend to vote Republican.
So, when you look at it from a sampling basis, women and blacks are typically over-polled relative to historical turnout rates. At the same time, men and older Americans are being under polled. This is clearly true in Kansas that has Greg Orman leading Republican Pat Roberts 45 to 44 percent. There, men were under polled 49% to 51% for women. Yet, historically, men have a higher turnout than women. Also 45% of those polled were under age 45; despite more older voters expected to show up. While it is true that sorting out the "likely voters" from those polled helps adjust for the demographic turn out rates, that action still doesn't take into account for the turnout rates of the individual groups mentioned above.
These are the reasons why I think that a Republican win is being grossly understated and why most of those "too close to call" races are probably more Republican than the Real Clear Politics averages suggest. Thus, expect Republicans to win on the high side of the predicted 5 to 8 seats of the 10 toss up races.
Real Clear Politics: 2014 Senate Races: http://www.realclearpolitics.com/epolls/2014/senate/2014_elections_senate_map.html
Voter turnout always drops off for midterm elections, but why?: http://www.pewresearch.org/fact-tank/2014/07/24/voter-turnout-always-drops-off-for-midterm-elections-but-why/
How the Rise of Reagan Seniors Helps Republicans in November: http://www.businessweek.com/articles/2014-09-25/reagan-loving-senior-voters-give-republicans-an-advantage-in-midterm-elections-and-maybe-control-of-congress
2014 Midterm Elections: More Women In The Electorate, But More Men Vote: http://www.ibtimes.com/2014-midterm-elections-more-women-electorate-more-men-vote-1712601
Women More Likely to Be Democrats, Regardless of Age: http://www.gallup.com/poll/120839/women-likely-democrats-regardless-age.aspx
Obama looks to black radio listeners to boost (black) Dem votes in midterms: http://www.theind.com/news/indnews/19331-obama-looks-to-black-radio-listeners-to-boost-dem-votes-in-midterms
Larry Sabato's Crystal Ball: Current outlook: Republicans net 5-8 Senate seats: http://www.centerforpolitics.org/crystalball/2014-senate/
Kansas Senatorial Race: NBC/Marist Poll: http://maristpoll.marist.edu/wp-content/misc/KSpolls/KS141018/Complete%20October%2026,%202014%20Kansas%20NBC%20News__Marist%20Poll%20Release%20and%20Tables.pdf