For evidence that maintaining their Senate majority could get tricky for Republicans, look no further than the predicament of Ohio's Rob Portman, one of the most respected GOP voices in the upper chamber. He's the type of Republican that official Washington respects: experienced, pragmatic, and politically astute. He was on Mitt Romney's short list for vice president, is one of only four Senate Republicans to support gay marriage, and comfortably won in the GOP's 2010 electoral rout.

But in a sign that politics play much differently in Ohio than Washington, Portman is one of the more vulnerable Republican senators for reelection—and party leaders are growing increasingly nervous about his prospects.

The latest Quinnipiac survey shows him trailing his likely Democratic opponent, former Gov. Ted Strickland, by six points—the second straight poll to show him behind. His name identification back home is surprisingly low, at just 64 percent, according to the poll.

Republican officials worry that those weak numbers are a consequence of not spending enough time in his home state, missing town-hall forums for fundraising duties as the National Republican Senatorial Committee's vice chairman last cycle. Between 2013-2014, Portman made 10 fundraising trips to New York alone, according to his campaign, at the expense of spending valuable time in Ohio.

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"He did far too much for the committee at his own expense," said one close GOP ally of Portman's. "We're going to have a very competitive race in Ohio next year."

Portman's campaign is a revealing bellwether in the battle for the Senate. If he can't hang on to a seat that once looked awfully secure, Democrats will have a good chance to reclaim the majority. And if the attributes that make Portman an effective senator aren't political assets, other senators in battleground states can't take anything for granted.

A Portman loss, combined with GOP defeats in more-liberal Illinois and Wisconsin, would mean Democrats would only be one seat away from a 50-50 Senate split—with opportunities in Florida, New Hampshire, and Pennsylvania still on the board. But if Portman prevails, Democrats would probably need a clean sweep everywhere else to retake control.

Portman certainly isn't taking his reelection for granted. He stockpiled more than $8 million in preparation for the campaign, making him the best-financed Republican senator facing a competitive reelection in 2016. His advisers argue that Ohio's sheer size makes it difficult for any politician to have universal name identification, and that early ad buys will quickly improve his standing in the state. They point to his still-solid overall favorability numbers of 43-21 in the Quinnipiac survey. They're confident that Strickland's economic record as governor gives them plenty of fodder for attack ads.

That's the optimistic view.

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But the more hardheaded view of Portman's prospects is that he's vulnerable to forces outside of his campaign's control. At a time when anti-Washington sentiment is near historic highs, his political experience and pragmatism aren't the selling points they once were. Meanwhile, the country's growing polarization means that voters are increasingly casting straight party-line ballots instead of closely assessing the merits of individual candidates. Control of the Senate is dangerously dependent on the national environment, regardless of how well the GOP's candidates run.

If those trends continue, Republicans could find their majority at risk. All politics is no longer local; each election more closely resembles a national referendum. If Hillary Clinton wins the presidency, she's well-positioned to bring the Senate along with her.

For this cycle, the map is difficult for Republicans, who are defending many more seats than their Democratic counterparts. Of the nine most-competitive Senate seats, seven are held by Republicans—and six feature sitting Republican senators. Eight of the races are being held in states that President Obama carried twice.

Democrats already are confident that they'll pick up two GOP-held seats in friendly territory. Even though Sen. Mark Kirk has a long track record of winning bipartisan support, national Democratic polling shows that he's already running well behind Rep. Tammy Duckworth. In a race where he has little margin for error, Kirk has gotten in trouble with gaffes that have drawn fire from Democrats and ridicule in the Chicago press.

Democrats also are bullish on their chances in Wisconsin, where public polling shows former Sen. Russ Feingold ahead of Sen. Ron Johnson in a rematch. Johnson is facing a double-whammy: After running an effortless campaign as the outsider against Feingold, he now needs to defend his own record in Washington. Meanwhile, he has to contend with the liberal leanings of Wisconsin's electorate, which hasn't voted for a Republican presidential candidate since 1984.

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Johnson's reelection underscores the other predicament for Senate Republicans in 2016: The same class of candidates who railed against the establishment to win office in 2010 are now veteran politicians trying to figure out how to recast themselves as outsiders a second time around.

Democrats are all too familiar with circumstances being out of their candidates' influence. Just last year, despite trumpeting how rare it was for incumbents to get knocked out of office, four Democratic senators lost their seats in the GOP wave. Even though these candidates were personally well-liked back home, they couldn't distance themselves from the national party's poor standing in their respective states. It didn't even matter that most of them were brand-name quantities in their home states—with names such as Udall, Pryor, and Landrieu.

There's one important difference between 2014 and 2016, however. Last year, red-state Democrats could do little to convince President Obama to move to the middle for their own best interests. On myriad issues, from health care to energy and national security, Obama's positioning was well to the left of the electorate's political sweet spot. With higher turnout expected from core Democratic groups and a battleground map fought in friendlier states, Democrats are hoping 2016 will be a different story.

For targeted Republicans such as Portman, their biggest hope is that their party nominates a presidential candidate with broad general-election appeal. That, more than individual campaign tactics, will set the stage for the Senate landscape. It's a sobering reminder for an accomplished candidate like Portman—whose resume includes stints as senator, President Bush's U.S trade representative, and leading the Office of Management and Budget—that his fate lies more in the hands of Hillary Clinton's campaign than his own.

CORRECTION: An earlier version of this story misstated the number of fundraisers Portman attended in New York City. Portman made 10 fundraising trips to New York City between 2013-2014 in his role as National Republican Senatorial Committee vice chairman.

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