Sen. Amy KlobucharAmy Jean KlobucharOvernight Regulation: EPA sued over water rule delay | House passes bill to ease ObamaCare calorie rule | Regulators talk bitcoin | Patient groups oppose FDA 'right to try' bill Overnight Finance: Senators near two-year budget deal | Trump would 'love to see a shutdown' over immigration | Dow closes nearly 600 points higher after volatile day | Trade deficit at highest level since 2008 | Pawlenty leaving Wall Street group Dem senator presses FTC to ramp up Equifax hack probe MORE (D-Minn.) is the most popular senator running for reelection, while Joe Lieberman (I-Conn.) is the least, according to new polling data.

Public Policy Polling (PPP), a Democratic-leaning firm, looked at the approval ratings for 59 senators during the second half of 2010, and several of them will face voters in 2012. The senators were ranked by the spread between their approval numbers and disapproval numbers.

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Of the 59, Klobuchar is the most popular, with an approval rating of 59 and a disapproval of 29 (a 30-point spread), while Lieberman has an approval rating of 33 and a disapproval of 54 (a 21-point spread).

Lieberman hasn't said what he'll do in 2012, but Klobuchar is preparing for rivals, including a possible challenge from Tea Party darling Rep. Michele BachmannMichele Marie BachmannBachmann won't run for Franken's Senate seat because she did not hear a 'call from God' Billboard from ‘God’ tells Michele Bachmann not to run for Senate Pawlenty opts out of Senate run in Minnesota MORE (R-Minn.).

Coming in second was Sen. Scott Brown (R-Mass.), who's also up in 2012. He had a 53 percent approval and 29 percent disapproval (a 24-point spread). Sen. Judd Gregg (R-N.H.), who is retiring, was third, and Sen. Olympia Snowe (R-Maine), also up in 2012, was fourth, with a 56 percent approval and 34 percent disapproval (a 22-point spread).

Of other senators up in 2012, near the bottom were Sens. Claire McCaskillClaire Conner McCaskillGovernment watchdog finds safety gaps in assisted living homes GOP Senate candidate fundraising lags behind Dems in key races McCaskill challenger links human trafficking to 'sexual revolution' of 1960s MORE (D-Mo.) — 43 approval/44 disapproval; Bob CaseyRobert (Bob) Patrick CaseyDem senator: Pence all 'talk, no action' GOP Senate candidate fundraising lags behind Dems in key races Overnight Health Care: Senate Dems block 20-week abortion ban | Azar sworn in as HHS chief | Dems demand answers on family planning funds | GOP takes sting out of ObamaCare MORE Jr. (D-Pa.) — 36 approval/40 disapproval; and John Ensign (R-Nev.) — 41 approval/48 disapproval.
   
Retiring Sen. Roland Burris (D-Ill.) came in last, with a 19 percent approval rating and a 58 percent disapproval rating.

PPP's Tom Jensen notes in his analysis that "the average approval rating for a senator right now is 43 percent approving to 40 percent disapproving" and that "there's very little difference in the average approval of Republican senators versus Democratic senators." The average GOP senator had a 43/37 split, while the average Democrat rated a 44/41. 

Several senators up for reelection came in under the 50 percent approval mark, including: Sens. Ben CardinBenjamin (Ben) Louis CardinMenendez to regain spot as top Foreign Relations Dem US could reinstate security assistance if Pakistan takes 'decisive' steps Cardin files to run for third term MORE (D-Md.), Kay Bailey Hutchison (R-Texas), Tom CarperThomas (Tom) Richard CarperOvernight Regulation: EPA sued over water rule delay | House passes bill to ease ObamaCare calorie rule | Regulators talk bitcoin | Patient groups oppose FDA 'right to try' bill Dem senator questions EPA on stark decline in grant awards Green group backs Sens. Baldwin, Nelson for reelection MORE (D-Del.), Jon Kyl (R-Ariz.), Jim Webb (D-Va.), Kirsten GillibrandKirsten Elizabeth GillibrandBritish health secretary fires back at Trump over universal health care claims Trump on Dems’ ‘universal' health-care push: ‘No thanks’ Gillibrand calls for DOJ to investigate US Olympic Committee over abuse scandal MORE (D-N.Y.), Sherrod BrownSherrod Campbell BrownTrump accuses Dems of 'treasonous' behavior Former Ohio football star faces conservative rival in GOP primary fight Dems press Trump for 'Buy American' proposals in infrastructure plan MORE (D-Ohio), Bill NelsonClarence (Bill) William NelsonInterior head to travel to Carolinas to discuss off shore drilling Overnight Finance: Trump touts trade agenda in State of the Union address | Consumer Bureau ruled constitutional | Fed leaves rates unchanged Green group backs Sens. Baldwin, Nelson for reelection MORE (D-Fla.), Maria CantwellMaria Elaine CantwellGreen group backs Sens. Baldwin, Nelson for reelection The US is falling behind in artificial intelligence research WHIP LIST: Shutdown looms as Senate lacks votes to pass House spending bill MORE (D-Wash.) and Debbie StabenowDeborah (Debbie) Ann StabenowDems press Trump for 'Buy American' proposals in infrastructure plan Kid Rock: Al Franken shouldn't have resigned Michigan State president resigns amid sexual abuse scandal MORE (D-Mich.).