That’s true even from friendly polls. Mark Pryor was so popular in 2008 that Republicans didn’t even field a challenger to him in Arkansas for his US Senate seat, which he had won from Republican Tim Hutchinson by eight points in 2002.  He defeated a Green Party candidate by an 80/20 margin with Barack Obama at the top of the ticket in the 2008 election, even though John McCain bested Obama in Arkansas by 20 points. Now, however, Pryor has become the most endangered Democratic incumbent in the upper chamber, with re-elect numbers closer to his last Green Party challenger than his own 2008 performance:

Sen. Mark Pryor (D., Ark.) could be facing a far tougher reelection bid than he might have thought. Just 37 percent of likely voters aid the Arkansas Democrat deserves to be reelected in a new poll conducted by Magellan Strategies BR.

Nearly half (47 percent) of likely voters aid it was time for a new person to be in the seat, while 28 percent said he should “definitely” be reelected.

The two biggest issues for Pryor in Arkansas are the two biggest legislative victories for the Obama administration — ObamaCare and the 2009 stimulus package.  Two-thirds of potential voters told Magellan, a Republican polling firm, that they are less likely to vote for Pryor due to his support of both bills.  It’s not hard to argue in either case that Pryor could have easily stopped both by standing his ground as a conservative Democrat rather than rolling over for Obama and Nancy Pelosi, and that’s almost certain to be the Republican attack strategy next year.

Of course, that’s what a Republican polling firm finds.  How about a liberal poll?  A poll commissioned by public-employee union AFSCME finds Pryor in front of presumed contender Representative Tom Cotton, but in deep trouble nonetheless:

Sen. Mark Pryor (D-Ark.) has an 8-point lead over his likely Senate challenger, Rep. Tom Cotton (R-Ark.), in a new poll conducted for the pro-Democratic union AFSCME.

Pryor leads Cotton, who is expected to announce his Senate bid on Tuesday, by 43 to 35 percent in the poll.

The Hill includes this disclaimer:

Partisan polls should always be viewed with some skepticism, however, and this poll seems to skew too Democratic. President Obama’s approval rating in the state stands at 41 percent with 56 percent disapproving despite the fact that he only won 37 percent of the state’s vote in 2012 and has seen his approval numbers decline somewhat nationwide since.

Even at face value, though, this poll isn’t good news for Pryor.  He’s been in the Senate for 12 years and has tremendous name recognition in Arkansas; Cotton has been in the House for six months and has never run in a state-wide race.  The House race in November was his first political bid, which is why half of the poll’s respondents don’t have an opinion on Cotton at all. Any incumbent with a 43% rating against a relative unknown is in deep trouble, and that’s especially true of Pryor in a state as red as Arkansas.

Washington Post analyst Sean Sullivan wrote last week that Cotton’s entry into the race makes Pryor’s predicament go from bad to worse. Cotton will unite the ideological and pragmatic wings of the GOP:

Here’s what makes Cotton such a potentially formidable contender: He is popular among both conservative groups who prize ideology and establishment Republicans who worry about electabilty.

All too often for the GOP, it’s been an either/or question. In Indiana last cycle, moderate incumbent Richard Lugar would have cruised to reelection. But conservatives backed his opponent Richard Mourdock, whose controversial comments about rape toward the end of the campaign propelled Democrat Joe Donnelly to an unlikely victory. Republican losses in the past two cycles in Missouri, Delaware, Nevada and Colorado fit the same mold.

“Representative Cotton is a conservative leader and rock star candidate,” said Steven Law, the President of the GOP-aligned group American Crossroads. “Arkansas is now one of the very top pickup opportunities for Republicans this cycle and we are excited to get engaged in the race on behalf of Rep. Tom Cotton.”

Law hails from the wing of the GOP concerned about nominating unelectable candidates. He heads the Conservative Victory Project, an effort that aims to weed out candidates who are not good general election fits.

Conservative groups like the Club For Growth and the Senate Conservatives Fund — which, like Crossroads, matter because they spend money in congressional races — are also expected to get behind Cotton.

It won’t be a walkover for Cotton, but he thumped a well-known state politician in November to win a district held by Democrats for most of the last century, so he knows how to campaign. Eric Ostermeier explains just how historic a Cotton victory over Pryor would be:

As Smart Politics reported in March, Cotton’s candidacy faces challenging odds on paper: only 17 House freshmen have been elected to the Senate over the last century, and just two during the last 40 years (Minnesota’s Rod Grams in 1994 and Kansas’ Sam Brownback in 1996).

That is what Cotton has going against him.

On top of that, there is this bit of history going for Pryor.

The Democratic Senator is coming off a 2008 campaign in which the Republican Party did not even field a candidate and Pryor was victorious by 59.1 points over the Green Party’s Rebekah Kennedy.

So what are the odds Pryor could come back six years later after facing no major party competition and then lose the 2014 general election?

It’s never happened to an incumbent before.

A Smart Politics analysis finds that no incumbent U.S. Senator has lost a general election race coming off a victory in which he did not face a major party opponent.

Not yet, anyway.

Update: I wrote Alabama in one spot where I meant Arkansas. I’ve fixed it; sorry for the error.