From Cook’s latest in National Journal:

For much of this year, it seemed a near mathematical impossibility that Republicans could score the 10-seat net gain needed to flip the Senate, which is split between 59 Democrats (including two independents who caucus with Democrats and largely vote with the party) and 41 Republicans. As recently as six weeks ago, I wrote in a CongressDailyAM column that a GOP win was “certainly possible” but “still fairly unlikely.” Although the “fairly unlikely” part is still valid, the possibility of a GOP takeover is growing.

To be sure, a 10-seat gain for Republicans remains hard. Eighteen Senate seats could plausibly turn over — a dozen held by Democrats and six by Republicans. Looking first at the five open seats — Florida, Kentucky, Missouri, New Hampshire, and Ohio — that the GOP is defending, the Republican challenger holds the lead in each race. Granite State voters won’t select nominees until September 14, but former Attorney General Kelly Ayotte, the Republican with the best chance of defeating Rep. Paul Hodes, the presumptive Democratic nominee, is increasingly favored to win the GOP nod. None of the Republican leads in these five states is insurmountable, but at this point, you would rather be the GOP nominee than the Democratic one in each place.