But interviews with key Democrats who are advising all levels of their party's campaign apparatus and with Republican officials suggest the big "if" that Cook worries about is a wild-card issue or event that could give the GOP a winning hand. Maybe not enough to avoid losses in the House and Senate and among the governorships, but possibly enough to keep both chambers in GOP control.
One of the big ifs has to do with the generic voter-preference polls that have shown Democrats with a strong advantage over Republicans all year. As reported in an earlier column, the latest Gallup Poll showed that edge shrinking to a statistical tie -- a poll virtually ignored in the Labor Day news stories. But even if the generic poll showed a continued Democratic advantage, it may not be enough in a number of GOP districts. The reason: Pollsters acknowledge that generic surveys have long skewed the results against the GOP by anywhere from 6 to 10 points.
Republican campaign officials told me that if Democrats are leading in the generic polls by 6 points or so, they feel confident they can repel the Democratic assault on the House.
"We look at the individual races and we find in our polling data that the Republicans are in very good positions," said Carl Forti, chief spokesman for the National Republican Congressional Committee.
"Republicans will be in the majority in the next Congress," he told me. The Democrats, though, are not making any such claim for their party. "We're not predicting we are going to take over the House," said Bill Burton, chief spokesman for the Democratic Congressional Campaign Committee.
The tightening of the generic numbers is one reason for the Democrats' reticence. But other factors have created self-doubts about their chances: One is the GOP's superior voter-turnout ground game and the higher redistricting walls they have built since the 2000 census to protect their incumbents. "The way the congressional district lines have been drawn (around GOP-held House seats) is going to be a challenge," Burton admitted.
As for those tightening voter-preference polls, Democratic officials told me they have seen similar movement in their internal surveys.
"What you are seeing is, yes, some Republicans are coming home in certain districts and there is some firming up of the Republican vote," another prominent Democratic adviser told me.
There "aren't as many vulnerable Republicans now as a month or more ago," he said.
White House Confirms James Foley Execution as First ISIS Attack on The United States | Katie Pavlich