Mitt Romney and President Obama are neck and neck in Missouri, according to a new poll from the nonpartisan pollster SurveyUSA.

In the poll, Romney leads Obama by a narrow 45-to-44 percent in a state that most consider to lean Republican and that is a must-win for Romney. Three late-July polls showed him with between a 6- and 9-point lead in the state, and neither candidate has shown any sign that he sees the state as competitive.

Half of the poll was conducted before Romney chose Rep. Paul Ryan (R-Wis.) as his running mate.

Sen. Claire McCaskill (D-Mo.) is in a worse spot than the president, according to the poll — she trails Rep. Todd Akin (R-Mo.) by 51 percent to 40. Two late-July polls had him with a much narrower, single-digit lead, though this poll's result could reflect a boost for him following his primary win last Tuesday.

The poll of 585 likely voters was conducted from Aug. 9-12 and had a margin of error of plus or minus 4.1 percent.