At National Journal’s Hotline on Call, Ronald Brownstein and Stephanie Czekalinski discuss the increasing importance of presidential candidate coattails in key races for the U.S. Senate. The authors explain:

This strengthening correlation between preferences in Senate and presidential races is now a key dynamic in the race for control of the upper chamber. (Read related story here.) The share of voters who split their tickets — voting for one party’s candidate for president, and the other’s for Senate — has steadily declined since the 1970s. Though there are exceptions in each cycle, that’s made it more difficult for senators, in effect, to get elected behind enemy lines in states that usually support the other party for president. That means the outcome of the presidential race will cast a huge shadow over the struggle for control of the Senate.
In recent years, it’s become routine for competitive candidates in Senate races to win over 80 percent of the voters who back their party’s presidential nominee. An array of Quinnipiac and Marist Institute/NBC/Wall Street Journal polls this month show virtually every competitive Senate candidate in both parties crossing that threshold in the states that were surveyed.

Brownstein and Czekalinski cite recent polling data showing a tightening relationship between support for President Obama on the one hand and the senate candidacies of Elizabeth Warren in MA, Chris Murphy in CT and Sherrod Brown in OH on the other, along with other Democratic candidates in close races in other states. The same is true for their Republican adversaries. The authors add,

The intensity of these relationships underscores the difficulty facing candidates hoping to swim against the tide of the presidential race in their state. That current is proving a huge obstacle for Republicans McMahon and Brown in states that Obama is on track to win comfortably. Conversely, GOP strategists say, they believe an anti-Obama current will benefit their candidates in North Dakota, Arizona, Montana and Indiana, states with hotly-contested Senate races that Obama is expected to lose.

There are a couple of exceptions. Most notably Claire McCaskill is outperforming the president in MO, owing no doubt in large part to Republican Todd Akin’s insipid remarks about ‘legitimate rape.’ (We may soon see a similar phenomenon in IN, where Republician Richard Mourdock has recently upped the ante on stupid statements about rape.
There are a few other races, where the coattail effect is somewhat in question. But overall, the authors conclude:

The general trajectory toward greater line party voting in Senate races has been extremely powerful. By 2008, the share of voters who backed a presidential candidate of one party and a Senate candidate of the other had dropped to 13 percent – less than half of the 28 percent who reported splitting their ticket during 1972 – the peak year, according to figures from the University of Michigan’s National Election Studies. In 2010, Democrats won the Senate races in nine of the ten states where Obama’s approval rating stood at 48 percent or above-and lost 13 of the 15 where only 47 percent or less of voters approved.

One wild card of 2012 which may actually enhance the coattails relationship is the Democrats’ reportedly more extensive ground game. And who knows, a reverse coattail effect may kick in as a result of the outrageous comments about rape from the likes of Mourdock and Akin, poisoning the GOP brand nationwide among women swing voters.