Just one week ago, the number-crunchers at FiveThirtyEight.com gave Democrats 68.9 percent odds the party will take control of the Senate as a result of Tuesday’s elections. The betting odds then were not that far from the peak likelihood predicted by the website: 74.6 percent, on October 18. Yet, in one of the most dramatic movements in this year’s political contests, Democrats, in the space of just one week, have lost their position as odds-on favorites to capture a Senate majority.
FiveThirtyEight now ranks the likely outcome a dead heat, leaning by the slimmest smidgen to the GOP. Republicans have a 50.7 percent chance of holding their Senate majority, while Democrats have a 49.3 percent chance of taking it away. Republicans haven’t led in the polling site’s Senate odds since the end of July, when the party was enjoying the last of its convention bounce. The question for most of the last three months hasn’t been whether Democrats would take the Senate, but how crushing the defeat of Republicans would be.
Republicans currently hold 54 Senate seats. If Hillary Clinton wins the presidency, Democrats will only have to take four seats from the GOP to control the upper chamber, as a Senate tie is broken by the vice president. Once upon a time, that outcome looked easy, especially since Republicans are defending more than twice as many Senate seats as Democrats are this year (a legacy of the blowout GOP victories in the 2010 election). Now, seats that the Democrats thought were a lock have shifted dramatically. As late as October 29, for example, the odds of Democrat Evan Bayh winning in Indiana were 72.9 percent to 27.1 percent. As of early Monday afternoon, those odds are almost exactly reversed, with chances for a Todd Young victory at 64.4 percent to 35.6 percent. (Note: the latest numbers reflect what is shown on FiveThirtyEight’s trend lines, not the top of the page—the odds can shift frequently within the span of a single day.)
The specificity of those odds is silly, of course, but what they do show is a significant trend in the poll numbers, a trend that is breaking across multiple toss-up races. On November 2, New Hampshire Democrat Maggie Hassan enjoyed a 64.5 percent to 35.5 percent lead in the odds over incumbent Senator Kelly Ayotte. As of today, those odds favor Ayotte, 55.5 percent to 44.5 percent. As of October 29, Democrat Jason Kander was the odds-on favorite in Missouri by 61.8 percent to 38.2 percent; by Monday, those odds had flipped, with Republican incumbent Roy Blunt jumping to a 58.6 percent to 41.4 percent lead in the odds of who will win in the Show Me State.
Things are even closer in Nevada, the one state where a Democrat seat is in play. (The race is to replace Harry Reid, who is retiring, and none too soon at that). Less than a week ago, betting odds put the chances of Democratic candidate Catherine Cortez Masto winning at 62.3 percent as opposed to 37.7 percent chance for the Republican. But now, her lead over the GOP’s Joe Heck has vanished—Heck now leads with odds of 52 percent to Cortez Masto’s 48 percent.
For there to be such lock-step movement in these estimates of the candidates’ chances across such different and diverse states clearly can’t be attributed solely to the candidates themselves. Some measure of a trend is at work—a trend that doesn’t appear to have been paralleled in the presidential polling. If so, it is a trend that will make the outcome of the Senate races the most interesting and closely contested results Tuesday night.
