Not since a Republican president named Richard Nixon was trying to explain away the Watergate scandal has the Grand Old Party been on such a losing streak in special elections for congressional seats vacated by Republican incumbents.

For the third time in four months, a Democrat has won a special election for a House seat representing a district that George Bush won overwhelmingly in 2000 and 2004 and where Democrats hadn’t had been out of the congressional competition for years.

This time, the Republican lost a U.S. House seat in a north Mississippi, where Democrat Travis Childers prevailed over Republican Greg Davis by a remarkably comfortable 54-46 margin.

For Republican congressional leaders and strategists who were already worried about the 2008 cycle, the Mississippi vote that was forced by Republican Governor Haley Barbour’s decision to appoint Congressman Roger Wicker to the Senate seat vacated by Trent Lott delivered a dismal result that just about everyone agrees bodes ill for Republican prospects in House races this fall.

The Hill, a Capital Hill insider newspaper, opined Tuesday night that the result sends “a listless GOP further into a state of disarray.”

Democratic Congressional Campaign Committee chair Chris Van Hollen, D-Maryland, was crowing that, “After three consecutive special election defeats in districts President Bush twice won easily, it is abundantly clear the American people have turned their back and shut the door on the special interest-driven agenda of the Republican Party. There is no district that is safe for Republican candidates because President Bush’s failed policies have hurt every community in America.

And a nonplussed Minority Leader John Boehner, R-Ohio, seemed to agree, admitting that the Mississippi result was a “wake-up call” to Republican candidates nationwide.

Democrat Childers scored the Mississippi win despite an ugly, racialized campaign by national Republicans and their special-interest allies, who knew that losing this race would undermine the party’s fund-raising and candidate-recruitment efforts going into a fall election cycle where the GOP was already vulnerable because of incumbent retirements, nearly empty bank accounts and a general sense of malaise.

The National Republican Congressional Committee spent $1.3 million — almost one fifth of the all the cash it had on hand — on this one contest. And after losing an Illinois special election for the seat vacated by former House Speaker Dennis Hastert and a Louisiana race for a seat that had been in GOP hands for three decades, they pulled no punches.

The GOP strategy was to link Childers, who is white, to Barack Obama and the Rev. Jeremiah Wright, black men who it was presumed would frighten southern voters.

It didn’t work.

Childers put some distance between himself and Obama. But he put even more distance between himself and the Bush administration.

When the Republicans made the bizarre decision this week to send Vice President Dick Cheney into Mississippi to campaign for Davis, Childers objected to Davis inviting “Big Oil’s best friend, Dick Cheney, to North Mississippi” and linked Cheney’s candidate to “the skyrocketing cost of gas.”

Childers ran as an old-school economic populist campaign, and he made opposition to free trade central to his appeal.

Touting a “No New Trade Deals” pledge, Childers campaigned as an aggressive critic of the Colombia Free Trade Agreement. And he promised not to support any new pacts “like NAFTA and CAFTA.”

The bottom line: Barack Obama may or may not prove to be a burden for Democrats seeking to win Republican-held House seats in Republican-leaning southern and western states this year. So far, he’s not proving to be much of a problem, and in some cases he may even help.

But George Bush’s economic policies — especially his advocacy for more free-trade pacts — is a major burden for Republican candidates. And Dick Cheney is the best GOP surrogate a Democratic challenger could ask for.