If there were any doubts that Sen. Mark Pryor (D-Ark.) is the most vulnerable senator in the country this election cycle, Rep. Tom Cotton (R-Ark.) appears to have put them to rest.

Rep. Tom Cotton (R-Ark.), above on the right. (Richard Rasmussen/AP)

Cotton will reportedly announce his bid against Pryor next week, giving Republicans a top recruit for a race they have been eyeing eagerly.

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.

"Run, Tom, Run!" wrote Club spokesman Barney Keller in e-mail, when asked for a reaction to reports that Cotton is running. The Club backed Cotton in his campaign for the House last year.

A spokesman for SCF did not immediately respond to a request for comment Wednesday, but the group has released polling and advertisements that have looked like efforts to spur Cotton to enter the mix.

On the other side stands Pryor, a Democrat fighting for his life in an increasingly red state. Pryor has been getting it from both sides. Conservative groups seeking to dislodge him have already lobbed attack ads his direction. Meanwhile, New York Mayor Michael Bloomberg's gun control group took him to task for voting against expanding background checks on gun purchases.

Pryor's first TV ad was a defensive spot, hitting back against Bloomberg and distancing himself from President Obama. It's never a good sign when a candidate's first commercial is a response to criticism.

"Like [former senator] Blanche Lincoln, Pryor faces tough primary and a tougher general," said National Republican Senatorial Committee spokesman Brad Dayspring.

Cotton has a lot to prove. He is new to Congress, and still relatively new to the world of campaigns. Democrats have already signaled they intend to paint him as an extremist, and he will have to combat those charges to survive. And it's not yet clear how the GOP primary field will ultimately look.

"Cotton votes in lock step with Washington special interests and against Arkansas, but still thinks that he deserves a promotion," said Democratic Senatorial Campaign Committee spokesman Justin Barasky, who called Cotton an "extreme ideologue."

All in all, if you're a Republican strategist of pretty much any stripe, you've got to be smiling at the news that Cotton is running. Against the backdrop of Senate map in which Republicans have virtually no margin for error in their pursuit of the Senate majority, Cotton's decision is a welcome development for the GOP.