One of the most vulnerable incumbents up for reelection in November, McCaskill faces an electorate disenchanted by President Obama and a group of conservative super-PACs eager to tie her to the Democratic president. Obama lags two points behind Mitt Romney in Missouri, according to the poll.

Another Democratic poll in January from Public Policy Polling showed McCaskill in a dead heat with her GOP opponents.

"You know the news is bad for McCaskill when she can’t even top 46 percent in a poll paid for by Democrats, conducted by Democrats, and touted by Democrats,” said Lloyd Smith of the Missouri Republican Party, noting that Majority PAC has already spent six figures on ads defending McCaskill.

McCaskill’s lead is even larger — more than 30 points against all of the Republicans — among those who self-identify as moderates, Mellman wrote in a polling memo.

Another bright spot in the poll for McCaskill is that her margin is largest — 9 points — against Steelman, whom polling shows is leading the pack in the Republican primary.

The poll of 600 registered voters was conducted May 8-10 using live telephone interviews and had a margin of error of 4 percentage points.

The Hill rates this race a toss-up.

- This post was updated at 12:16 p.m.