With just four months to go until the first 2008 presidential balloting, the clock is quickly ticking on the political life of the second-tier White House candidates, and it appears that Congress will play host to the race?s first casualties.

In fact, speculation already is mounting over which of the 10 Congressional presidential hopefuls will be first to drop out of the hunt, with rumors circulating that at least one early withdrawal may be imminent.

The chatter is loudest in the Democratic field, where talks of an opt-out within the coming weeks by Sens. Joseph Biden (Del.) or Chris Dodd (Conn.) are growing, while the leading candidates, Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.), continue to command the spotlight and shore up their war chests. Republican pullouts remain less predictable given the party?s uncertainty over who will ultimately secure and hold frontrunner status.

?This thing has become so fait-accompli that it?s Clinton and Obama or Edwards, that I don?t know how these guys stay in,? said one Democratic political operative who has worked on numerous presidential campaigns. ?There?s no oxygen left. I think you?ll see the first casualties after? the Sept. 30 fundraising deadline.

The close of the month marks the end of the third financial quarter for the cycle, viewed as the next critical ? and perhaps campaign-ending ? benchmark in the race for the White House. So far, the lower-tier Democratic candidates have struggled to keep pace with Clinton, Obama and Edwards, while several Republican hopefuls have failed to keep up with former Massachusetts Gov. Mitt Romney and ex-New York Mayor Rudy Giuliani.

?I wouldn’t be surprised if one or two drop out after the September numbers are released,? offered a second Democratic political operative. ?It?s a tough road for the tier-two guys. I just don?t know how they hang on.?

Still, six Senators and four House Members remain in the crowded hunt for the presidency. Of those, just three ? Clinton, Obama and Republican Sen. John McCain (Ariz.) ? are seen as truly viable.

Also in the race are conservative GOP Sen. Sam Brownback (Kan.) and four long-shot House Members: Democratic Rep. Dennis Kucinich (Ohio) and GOP Reps. Tom Tancredo (Colo.), Duncan Hunter (Calif.) and Ron Paul (Texas). Although none of those five has a serious shot at the nomination, many observers believe Kucinich and Paul won?t opt out of the race under any circumstances.

Kucinich proved that in 2004 when he continued in the race until the July nominating convention, even after the Democratic nominee, Sen. John Kerry (Mass.), had long secured his party?s nod.

Despite lagging in the money chase and chatter to the contrary, Biden insisted that his campaign isn?t going anywhere until after the Iowa caucuses on Jan. 14, a day he views as make-or-break for his campaign. Biden has been shoring up endorsements and funneling staff to that early primary state, and he vowed this week that he has enough money to sustain his operation until then, and afterward, depending on the Iowa outcome.

?I still believe I can win this,? Biden said. ?I?m going through all the early caucuses and primaries.?

But, he said, ?If I don?t come up fourth, fifth or sixth [in Iowa], then I?m done.?

Brownback, who also has had difficulty gaining traction, views Iowa in a similar vein, having put most of his stock in finishing among the top four candidates there. Like Biden, Brownback lags significantly behind in his party?s presidential field.

Yet the Kansas conservative said he too plans to stick it out through mid-January, believing that a strong finish in Iowa ?gives us a real shot.?

?You build momentum by performing,? Brownback said this week. ?The next place to perform is the Iowa caucus, and that?s what I?m focused on.?

Although the goal is clearly the early states, even the bloc of Congressional presidential hopefuls acknowledges the increasing difficulty of competing in a heavily front-loaded primary calendar against major financial powerhouses such as Clinton, Obama, Romney and Giuliani.

Sen. Evan Bayh (D-Ind.), who earlier this year abandoned his own presidential ambitions in the face of low polling numbers and difficult fundraising, said he learned firsthand how difficult it can be to stay in the hunt. And, he added, the longer candidates campaign, the more difficult it becomes to tap into key donor pools.

?That?s the primary challenge that affects candidates in the second tier,? said Bayh, who recently endorsed Clinton and is viewed as a vice presidential prospect. ?More than anything, it?s just an enormous expense.?

?There are no rules for this,? added Sen. Joe Lieberman (ID-Conn.), who took a shot at the Democratic nomination in 2004 and had to drop out in February of that year.

Lieberman said it is impossible to predict the life span of any of the 2008 campaigns, but said: ?I think you have to presume everyone who is in the race will be in it through Iowa and New Hampshire and then the race will start to thin after that.?

A narrower field is a certainty, and despite the speculation to the contrary, none of the candidates in either party, in either chamber, seems poised for an immediate exit.

Rep. Ron Paul (R-Texas), whose Libertarian bent has caught grass-roots fire and fueled his fundraising, said he feels good about his position heading into the close of the fundraising period. In fact, Paul said that Monday was one of his best online fundraising days ? his campaign brought in around $167,000 through his Web site.

?We definitely did better than the second quarter,? Paul said Tuesday. ?And we always spend less than what we raise.?

Paul said he believed that the end of the third quarter would signify do-or-die time for some second- and third-tier contenders who have not been able to keep pace financially.

?Some people have less money coming in,? he said. ?I think that would be a big indicator.?

Indeed, the numbers will not be solid for many of the candidates, especially compared to the proven fundraising muscle of the frontrunners. Clinton and Obama already are boasting having raised another $20 million apiece in the third quarter, pushing them between $70 million and $80 million overall. Biden, for one, conceded this week he would ?continue to trail financially? heading into January.

Tancredo, another long shot who has tried to rally his campaign around a conservative anti-immigration stance, said he also didn?t fare as well as he?d hoped financially. Still, he shares the view that most of the candidates will stay in the race at least until the first primary votes are cast, pinning all of their hopes on their performance in Iowa and New Hampshire.

The Colorado Republican said that at the very minimum, a fourth-place finish in Iowa is needed for any of the hopefuls to remain viable. Between now and then, he said, some candidates might choose to stay in the contest and attend debates, even if they don?t campaign seriously.

?They just like the idea of being there,? he said.