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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. Quinnipiac surveys this month, for instance, show that in Ohio Democrat Sherrod Brown is winning 90 percent of Obama voters, and Republican Josh Mandel is winning 84 percent of Romney voters. In New Jersey, Democrat Bob Menendez wins 90 percent of Obama voters and Republican Joe Kyrillos wins 80 percent of Romney voters. The relationships are even stronger in two other tight battlegrounds. In Wisconsin, both Democrat Tammy Baldwin and Republican Tommy Thompson are winning exactly 89 percent of the voters supporting their party's presidential candidate. In Virginia, Democrat Tim Kaine attracts 89 percent of Obama supporters and Republican George Allen 87 percent of Romney voters. Recent Marist/NBC/Wall Street Journal surveys have found similarly close relationships between presidential and Senate attitudes in Wisconsin and Ohio. The only partial exception to this rule: the latest Marist survey in Florida found Democrat Bill Nelson winning almost nine-in-ten Obama voters, but Republican Connie Mack attracting only 75 percent of Romney supporters. 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. The biggest exception to this pattern this year is in Missouri, where the incendiary comments about rape by Republican nominee Todd Akin has allowed Democrat Claire McCaskill to maintain a lead in a state where Obama appears headed for a thumping defeat. Democrats are now hopeful that comparably incendiary comments about rape this week by Republican Senate nominee Richard Mourdock in Indiana might allow them to engineer a similar reversal; until Mourdock's comments at a debate this week, private Republican polling had shown him opening a small lead over Democrat Joe Donnelly largely by consolidating support among Romney voters. Over the years, some senators have been able to buck these trends and win in states where the political climate toward the president in their state favored the other party, including North Dakota Democrat Byron Dorgan in 2004, Maine Republican Olympia Snowe in 2006, and West Virginia Democrat Joe Manchin in 2010. But such survivors increasingly appear to be exceptions. This year, Republicans are most optimistic that they could survive a narrow Romney loss and still win the Senate seats in Wisconsin and Nevada, where Republican Dean Heller has been leading Democrat Shelley Berkley. Beyond McCaskill, Democrats are most optimistic that Kaine in Virginia and Jon Tester in Montana could overcome an Obama loss in their state; after this week's imbroglio, they also hold out hope for Donnelly in Indiana. But the Romney tide in the latter two states will be extremely challenging to overcome. And even in Virginia, Kaine probably can't swim very far upstream. The latest Quinnipiac survey there shows Kaine winning 10 percent of Romney voters while Allen is attracting only 7 percent of Obama voters, a differential that could prove crucial in a very narrow presidential result but couldn't reverse a wider one. 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.