When Missouri Republican Senate candidate Todd Akin made his now-infamous comments about "legitimate rape," it wasn't just an epic disaster for his campaign, but also a huge payoff on a Democratic investment.

That's because Akin was the Democrats' choice as the GOP nominee. They spent at least $1.5 million in ads during the primary (some sources say $2 million) to boost his campaign. They thought he was the weakest of the candidates running and therefore represented incumbent Sen. Claire McCaskill's best hope of survival. Democratic control of the Senate may hinge on this race.

McCaskill has been widely recognized as the most vulnerable Senate Democrat up for re-election. She won her seat in 2006, a bad year for the GOP, beating incumbent Jim Talent just 50 percent to 47 percent. In February, Public Policy Polling reported that she had only 42 percent job approval, with 49 percent disapproving.

Missouri is a "red state," though not as red as others. John McCain barely beat President Obama there in 2008, by about 4,000 votes out of nearly 3 million cast. Democrats knew they had a chance in 2012 if McCaskill drew a weak opponent.

The GOP primary was split three ways among Akin, former State Treasurer Sarah Steelman, a Tea Party favorite who was endorsed by Sarah Palin, and businessman John Brunner, who poured $8 million of his own money into the race. Who was the conservative? All three were. As one of the state's top Republicans told The Examiner on Tuesday, "You couldn't fit a piece of paper between the three of them on any issue."

But even if there was no ideological difference to speak of, Akin was especially weak. He trailed the two in most polls and had a history of comments that Democrats believed they could use against him. On a C-SPAN program last year, he said of Social Security: "I don't like it." As a six-term congressman, he also had a long trail of House votes to mine for negative ads.

In the GOP primary, the Democrats ran ads against Akin ostensibly meant to criticize his conservative stances. But the clear intention was to reinforce the idea among Republican primary voters that he was the most conservative candidate.

One radio ad said: "Todd Akin calls himself the true conservative, but is he too conservative?" A mellow voice over the sound of a folksy acoustic guitar then runs down a list of "Todd's" conservative stances.

It concludes: "So, it's no surprise Todd has been endorsed by the most conservative leaders in our country, Michele Bachmann and Mike Huckabee."

A person not paying close attention might have missed the very last line: "Paid for by the Democratic Senatorial Campaign Committee."

The reported $1.5 million spent on such ads is almost as much as the $2 million overall Akin has raised for his campaign. He won the Aug. 7 primary with just 36 percent of the vote. Brunner got 30 percent and Steelman 29 percent.

The Democrats' bet on Akin paid off better than they ever hoped with his bizarre comments on rape Sunday that appalled voters right, left and center. Now that he's become radioactive, the last thing Democrats want is for him to step aside. Asked if he should quit Monday morning on MSNBC, McCaskill said, "It's not my place to decide."

She then showed a touching concern for the feelings of GOP primary voters: "I really think that for the national party to try to come in here and dictate to the Republican primary voters that they're going to invalidate their decision, that would be pretty radical. I think there could be a backlash for the Republicans if they did that."

In Obama's press conference Monday -- called mainly to highlight the issue of Akin's comments -- the president was similarly agnostic on whether Akin should drop out of the race, despite having just termed his views "offensive."

"He was nominated by the Republicans in Missouri," Obama said. "I'll let them sort that out." But the Democrats' $1.5 million investment in Akin should entitle Obama to a say, shouldn't it?

Sean Higgins (shiggins@washingtonexaminer.com) is a senior editorial writer for The Washington Examiner. Follow him on Twitter at @seanghiggins.