This growing connection between presidential and Senate voting has implications that extend well beyond the 2014 battle for control. Red-state Democrats like Louisiana’s Mary Landrieu and West Virginia’s Joe Manchin, and blue-state Republicans like Pennsylvania’s Pat Toomey and Maine’s Susan Collins, are often the most eager Senate dealmakers. But the trend toward party-line voting means there will be fewer of them. With each party holding the presidential advantage in about half the states, it also means “the contemporary Senate will be very unlikely to see big majorities” for either side, and will likely experience more frequent switches in control, notes Emory University political scientist Alan Abramowitz.

Fewer dealmakers mean fewer deals; and narrow, tenuous majorities will encourage the minority party to wage war to deny the majority any accomplishments that could reinforce their fragile advantage. Both of those trends promise yet more Senate polarization. “It changes the internal dynamics,” notes Abramowitz. A Senate that more precisely tracks the red-blue divide is less likely to transcend it with creative compromise.

In an era of unstable majorities, whichever side controls the Senate majority after November will likely find its advantage fleeting. But these underlying changes are transforming the Senate more lastingly-and not for the better.