There is a theory in political science that says elections are often decided as far as six months to a year and a half in advance of election day. The reason? That's usually around the time when candidates decide whether or not to run. In potentially close races, the quality of the candidate can make all of the difference in the world. When discussing potential gains in the context of terrible Bush poll numbers, the war in Iraq, corruption, scandal and everything else that has happened in the past year, don't forget what happened a year and a half ago; we destroyed them in getting the candidates we wanted.
I'm going to focus on the Senate here, mostly because I know a lot more about it than I do about House races (see Superribbie or Steve Singiser for your House candidate needs). Entering the cycle, things didn't look particularly promising for Democratic chances in the Senate, mainly because there seemed to be a lot of vulnerable Democratic Senators. Then something funny happened; Chuck Schumer absolutely waxed the floor with Liddy Dole.
At the start of the cycle, the list of vulnerable Democrats was long. Maria Cantwell, Ben Nelson, Bill Nelson, Robert Byrd, Debbie Stabenow, Kent Conrad and Herb Kohl all appeared to be in danger. But one by one, Liddy Dole missed out on her man (or woman).
In Michigan, Debbie Stabenow is a one term Senator running in the face of a struggling economy and a fairly unpopular governor. She should be weak, and she would be, if either Mike Rogers or Candice Miller had gotten into the race. They are probably the two most promising politicians in the state, and either one could have given Debbie a stiff run for her money, but both balked at the risk, preferring to wait for Carl Levin to retire.
In Wisconsin, former Governor and Secretary of Health and Human Services Tommy Thompson could have made life tough for Herb Kohl, but opted not to run.
In North Dakota, Kent Conrad is extremely popular, but so is governor John Hoeven. There was concern that Hoeven would enter the race, but he announced early on that he wouldn't challenge Conrad.
In Nebraska, Ben Nelson sits in permanent danger as a Democrat in, well, Nebraska. Going into this cycle he seemed even more threatened because Former football coach/cheater Tom Osborne (Michigan would soooo have killed you in 1997), was considering making a run. Osborne probably isn't more popular than Jesus, but in Nebraska, it's at least debatable. However Osborne instead decided to run for Governor (a bid that would eventually fail in the primary).
In West Virginia, Robert Byrd is an institution. But he's also approaching 90, and WV has been drifting to the right in the past decade, leaving the great orator less strong than he once was. Shelly Moore Capito made noises about a run, but the DSCC raised early money for Byrd, scaring off the potential challenge.
Even where Dole got her candidate, things didn't go so well, as the "prized catch" of the cycle for Republicans turned out to be the Clinically insane Katherine Harris.
Contrast this with the Democratic side, where Claire McCaskill, Sherrod Brown and Bob Casey Jr. were all hesitant to take the plunge, but were convinced by Chuck Schumer that they'd be provided with what they needed to win. Add Sheldon Whitehouse Jon Tester, Harold Ford Jr., and the surprising emergence of Jim Webb and it becomes clear; Democrats just have the better candidates. In fact, it's difficult to think of a candidate the good guys didn't get. The closest they come to missing a chance is in Nevada, where Democrats probably could have found someone stronger than John Ensign, but just didn't seem to make it a priority. But overall, Democrats have by far gotten the better of the recruitment wars.
None of that was pre-ordained at the start of the cycle, and different candidates could have led to a vastly different result. So if Democrats beat Republicans on November 7th, keep this in mind; it's (in part) because Schumer beat Dole in the summer of 2005.