Walsh’s departure from the race came in the same week that two Republican senators — Pat Roberts in Kansas and Lamar Alexander in Tennessee — defeated tea party challengers in primary fights, ensuring that every GOP senator seeking reelection would be the party’s nominee.

These past seven days typified the fates of the two parties this election cycle. Democrats have been hit by retirements in tough states — Montana, West Virginia, South Dakota and, to a lesser extent, Iowa — and Republicans haven’t nominated the sort of extreme candidates who lack broader appeal in a general election.

Those realities — along with a national playing field in which a handful of incumbent Democrats are defending Republican-leaning seats in places where President Obama is deeply unpopular — have made a GOP takeover a better-than-50/50 proposition.