Until Sunday, Missouri Senator Claire McCaskill was a dead Democrat walking. Barack Obama is so unpopular in her state, she even planned to skip the party's convention lest it remind voters of her ties to the President and her past support for his agenda in Congress.

Ms. McCaskill's Republican rival, Todd Akin, has managed to revive her re-election hopes, however, with a jaw-dropping comment about a "legitimate rape" victim's ability to will away an unwanted pregnancy.

"If it's legitimate rape, the female body has ways to try to shut down that whole [pregnancy] thing," Mr. Akin told a Missouri television reporter in a clip that surfaced Sunday.

Story continues below advertisement

A race he seemed sure to win is now a toss-up.

Mr. Akin's blunder is just the latest example of Republicans snatching defeat from the jaws of likely victory. By choosing Tea Party adherents over moderate candidates, the party has seen its chances of winning other Senate races erode. Together with Mr. Akin's outburst, that has sapped the GOP's once-favourable odds of taking control of the Senate this fall.

"Akin really does create a problem for Republicans," said Jennifer Duffy, senior editor at the Cook Political Report, which rates the competitiveness of congressional races. "It is hard to find a path to a Senate majority for Republicans without Missouri."

Even if Mr. Akin decides to drop his candidacy, as several top Republicans urged him to do on Monday, it may be impossible for the party to contain the damage. Democrats seized on Mr. Akin's remarks to renew charges of a Republican "war on women" and link his views on abortion to those of GOP vice-presidential candidate Paul Ryan.

Republican nominee Mitt Romney called Mr. Akin's comments "insulting, inexcusable and, frankly, wrong." A Romney-Ryan administration, the Romney campaign added, "would not oppose abortion in instances of rape."

Democrats, however, underscored Mr. Ryan's long-standing opposition to abortion, with no exception in cases of rape and incest. While Mr. Ryan's views on social issues had been overshadowed by his hawkish fiscal positions, the Akin outburst has brought them into relief. That will force Mr. Ryan off his preferred economic turf on the campaign trail.

The Akin incident threatens to hurt Republicans most, however, in their bid to take control of the Senate after a six-year stint in opposition. With 23 Democratic seats up for re-election this fall – many of them in conservative states – the odds had favoured the GOP.

Story continues below advertisement

As recently as February, Ms. Duffy put the probability that Republicans would regain a Senate majority at 65 per cent. Now, the party has, at best, even odds of winning 51 seats, she thinks.

"There are fewer and fewer pick-up opportunities that Republicans can take for granted," agreed Nathan Gonzalez, deputy editor of The Rothenberg Political Report. "With the Missouri developments, let's just call it an extremely fluid situation."

Republicans now hold 47 Senate seats and need a net gain of four to control the upper chamber. While 60 seats are required to thwart a filibuster, a GOP Senate majority of fewer than 60 would still put the party firmly in the driver's seat in Congress. The GOP is expected to retain control of the House of Representatives, which it won in 2010.

Republicans had hoped to take the Senate in 2010 as Mr. Obama's approval rating sank and the national unemployment rate stood at 9.6 per cent. But the GOP primary victories of a handful of Tea Party candidates ended up helping Democrats retain control of endangered Senate seats in Colorado, Nevada, Delaware and Connecticut.

This year, the GOP's Tea Party wing helped oust Indiana Senator Dick Lugar, a 36-year centrist incumbent, in the party's May primary. That led Ms. Duffy to take the state out of the "safe Republican" category. Real Clear Politics now rates it a toss-up.

Senate seats not long ago considered likely Republican pick-ups or holds, including ones in Nevada, Florida and North Dakota, have also been placed in the toss-up category as GOP candidates fend off charges that they are too far to the right.

Story continues below advertisement

Meanwhile, Republicans are likely to lose a seat in Maine after the sudden retirement of moderate GOP Senator Olympia Snowe. And GOP Massachusetts Senator Scott Brown, who won the late Ted Kennedy's seat in a 2010 special election, is now in danger as consumer advocate Elizabeth Warren fights to win back the seat for Democrats.

Mr. Akin, currently a GOP congressman, won his Missouri Senate primary earlier this month over two more moderate candidates. That delighted Ms. McCaskill, who contributed to his victory by running an ad touting him as "the true conservative" in the GOP race.

No amount of advertising could have swung the Senate contest in her favour as decisively as Mr. Akin's comment about rape.

On Monday, he repudiated his "off the cuff" remark but reiterated his view that abortion should be illegal in all cases.

Despite his attempts at damage control, he left Republicans across the country reeling – and perhaps wondering how the party let a sure thing turn into such a mess.