One X-factor: GOP Sen. Pat Roberts in Kansas, whose race wasn't on either party's radar screen until recently. But his poorly run campaign and low approval ratings could make him vulnerable to deep-pocketed independent candidate Greg Orman, who has emerged as the de facto choice for Democrats. But if outside groups spend money and make the contest a referendum on partisan control of the Senate, odds are in Roberts's favor. It's still difficult to see Republicans sweeping red states, but losing in a state that Democrats haven't carried since 1932.
2. But in swing states, Democratic attacks have made their mark, giving them a fighting chance to maintain their majority.
At the same time, Democrats have been effective in raising the negatives of several of the GOP's leading recruits—Joni Ernst in Iowa, Cory Gardner in Colorado, and Thom Tillis in North Carolina. These are the contests that will determine if the sour public mood will lead to a Republican wave, and Democrats have relentlessly portrayed them as ideologically disconnected from the states they're running in.
In Iowa, the much-maligned Democratic nominee Bruce Braley holds stronger favorables than Ernst in both public and private polling—by portraying the state senator as too fiscally conservative for the populist state. Notably, in her latest ad, Ernst responds directly to accusations that she wants to privatize Social Security. It's a clear sign the attacks resonated. Tillis, whose fundraising has lagged behind expectations, has been hammered by millions of dollars in outside money casting him as too extreme for the state. He now trails Sen. Kay Hagan in public polling. And in Colorado, Democrats bet that women's issues would be particularly potent to use against Gardner, whose past vote for a "personhood" amendment restricting abortions has become the subject of numerous outside attacks. Gardner has been nimbly responding to the attacks, backtracking on his past positions and supporting over-the-counter access to contraceptives. But when you're playing defense—especially on social issues, a traditional Democratic strength—you're usually losing.
All three of these races are winnable for Republicans, thanks to the difficult national environment for Democrats. In all these races, the attacks have stunted the standing of the challengers, but haven't boosted the incumbents' numbers much. But they're also illustrative of how the damaged Republican brand is causing headaches for otherwise capable candidates in swing states. If Republicans come up empty in the traditional battlegrounds in a midterm election where most of the national indicators are in their favor, it's a glaring warning sign for their prospects in 2016.
3. There's a national wave if Republicans pick off two of the three battleground states: Colorado, Iowa, and North Carolina.