A new poll shows trouble for Democrats in the key battleground state of Missouri, with President Obama down 9 points to GOP candidate Mitt Romney and Sen. Claire McCaskillClaire Conner McCaskillHillicon Valley: Verizon, AT&T call off data partnerships after pressure | Tech speaks out against Trump family separation policy | T-Mobile, Sprint make case for B merger Senators introduce bipartisan bill to detect supply chain risks posing threats to national security Manchin becomes final Democrat to back bill preventing separation of immigrant families MORE (D-Mo.) trailing all three of her possible Republican opponents.

A new Mason-Dixon survey, conducted for the St. Louis Post-Dispatch and News 4, finds presumptive GOP nominee Mitt Romney leading President Obama 51-42 percent among registered voters. 

Obama holds a 51 percent unfavorable rating in the state with 34 favorable, challenging numbers three months from election day. Romney holds a 39 percent favorable rating to 27 unfavorable.

In the Senate race, the poll finds incumbent McCaskill trailing all three GOP primary contenders. McCaskill would lose a head-to-head matchup by nine points to businessman John Brunner (R), 52-41 percent, by eight points to former state Treasurer Sarah Steelman (R), 49-41 percent, and by five points to Rep. Todd Akin (R), 49-44 percent.

The poll also found McCaskill with a net unfavorable rating and faring poorly among independents. McCaskill’s numbers are underwater with 44 percent unfavorable to 37 favorable.  Among her possible opponents, Steelman has the highest unfavorable rating at 14 percent.

Among independent voters, McCaskill would lose in a matchup with Brunner 57 percent to 36, a 21-point edge for the GOP candidate. Against Steelman and Akin, McCaskill trails among independents by 16 and 11 percent respectively.

Republicans have sought to tie McCaskill to President Obama, who carried the state in 2008, but is facing a tough fight against Romney. A WeAskAmerica poll released this week put Romney ahead of Obama 49 to 40 percent, which equals the margin in the Mason-Dixon poll.

Mason-Dixon pollster Brad Coker said that McCaskill’s ties to the president were hurting her in the red-leaning state. “A lot of people are going to hold her accountable for her close ties to the president,” he said to the Post-Dispatch.

McCaskill is a top target for Republicans with outside groups signaling that they will spend heavily against her. 

Vice President Joe BidenJoseph (Joe) Robinette BidenElizabeth Warren can unify Democrats and take back the White House Giuliani doubles down on Biden comments: 'I meant that he’s dumb' Meghan McCain shreds Giuliani for calling Biden a 'mentally deficient idiot' MORE attended a private fundraiser for McCaskill in the state earlier this month, calling her the “toughest, most principled person in the United States Senate.”

McCaskill’s campaign announced in July that they had raised $2.6 million in the last quarter, a strong total that will help her remain competitive. 

Brunner led both Steelman and Akin in the poll among likely GOP voters. Brunner had 37 percent support to 27 percent for Steelman and 17 percent for Akin. The GOP Senate primary is Aug. 7

The poll of 625 likely Missouri voters was conducted July 23 to 25 and has a 4 percentage point margin of error.