Todd Akin is still a candidate for the United States Senate.

If that statement is still true in 18 days time, then it will also be true on Election Day. Nearly three weeks after the Missouri Republican congressman remarked in an interview that "legitimate rape" rarely causes pregnancy, the congressman has shown no signs that he is ready to yield to the many Republicans who want him to end his campaign. The clock is ticking, and if Akin doesn't drop out soon, he will appear on the ballot as the GOP nominee -- no matter what.

So what are some reasons Akin might drop out? It's not a question with a clear answer. And that's a large part of the problem for Republicans pressuring him to step aside.

"I've asked him the same question, and there are none," said Rick Tyler, a former spokesman for Newt Gingrich who joined Akin's campaign after the congressman stoked controversy with his comments.

In the weeks since he landed on the national radar following his controversial interview, Akin hasn't appeared to waver in his commitment to remain the Republican nominee. "Let me make this absolutely clear. I am going to November, and I am planning to beat Claire McCaskill," he recently said. "Not because it's about me, but first of all the people of this state voted and I don't believe it's right for a couple of party bosses to chase somebody out and plug in who their pets are. I don't think you want to do that to this country. And the second thing is when people voted for me, I take that very seriously."

So far at least, there are some factors that don't look like they would spur him to make an exit. One is criticism from within his own party. He's received tons of it. From almost everywhere. And he isn't going to receive the support of influential outside groups aligned with the party.

Polling does not appear to have swayed Akin to leave the race either, as public polling has shown support for him has sharply declined. Nor does money appear to be a deterrent. Without outside group support, Akin's reach is limited. (He had far less money in his campaign account than Sen. Claire McCaskill (D) according to campaign finance reports from late in the summer.)

And the lack of funds could be taking its toll on him. The CBS affiliate in St. Louis reported on Friday morning Missouri TV stations have been pulling Akin's ads because he hasn't been paying for the airtime. Tyler said that no ads have been canceled, and the campaign is "paying as we go."

But even if Akin can stay afloat on the airwaves, he is still facing an uphill climb. Tyler said the campaign has raised $400,0000 online since Aug. 19. But McCaskill has millions in the bank and can count on the support of more from national Democratic groups.

Akin is guided heavily by his Christian faith and a close circle of advisers. Those who know Akin say the input of his family is important. His son manages his campaign, and his wife is said to be very influential. When it comes to potentially changing his mind, these factors may be the most important ones.

"The only way he gets out is if his wife, Lulli, is either convinced they have no money and will not get any, or some of his core supporters suggest Todd get out," said one Missouri GOP strategist not aligned with Akin.

Akin has until Sept. 25 to end his bid via a court petition, clearing the way for state Republicans to choose a replacement for him. If he doesn't step aside by then, his name will appear on the ballot.

For Democrats, the odds of victory in November increase if Akin is the GOP nominee. For her own part, McCaskill has been focusing on positive campaign message. She released a new TV ad on Friday pitching herself as a centrist. The spot does not mention Akin.

Tyler said that the Akin campaign is hopeful Republican groups will gravitate back to the race if he shows strengths in the polls. But until the 25th, the strategists running those groups will hold out hope that Akin will, for one reason or another, step aside for the good of the party.

It's just difficult to determine what such a reason might be.