Ron Brownstein writes at National Journal:
At the core of the Democrats’ surprising pickup of two Senate seats was a consistent pattern. In almost every major contested Senate race, exit polls showed that the Democratic candidate won more support among voters who also backed President Obama than the Republican nominee did among voters who backed Mitt Romney.
In two sides of the same coin, that means almost all major Democratic Senate candidates did a better job than their Republican rivals of unifying their base and attracting more crossover voters. That pattern allowed Democrats to virtually sweep the Senate races in the states Obama that won and to triumph in four states that Romney carried decisively—Indiana, Missouri, Montana, and North Dakota.Indiana and Missouri, of course, were outliers: if not for catastrophic gaffes, Republicans would have won those races. But the crossover phenomenon did clearly tip Montana and North Dakota.