By Bob Benenson, CQ Staff

The breathtaking sweep of the Democrats’ victories in Campaign 2008 almost certainly could not have been accomplished without the emergence of Barack Obama , a galvanizing force whose drive for the White House stirred fervor among party activists and drew millions of new voters to the polls. The historic nature of his election as the nation’s first African-American president is magnified by the unexpected margins by which he outran Republican John McCain in both the popular and electoral votes.

Senate Majority Leader Harry Reid, House Speaker Nancy Pelosi and about 1,000 Democrats celebrate at the DCCC/DSCC Watch Party in Washington. Click here for video

Nor could the hefty expansion of the Democrats’ majorities in the Senate and the House over this and the 2006 election cycles have occurred without the rise of a cadre of tough-minded campaign strategists, who steered the party to a gain of at least five Senate seats — with three races too close to call — and more than a dozen House seats this year, also with several additional gains hanging in the balance.

Democratic National Committee Chairman Howard Dean, Senate campaign chairman Charles E. Schumer of New York and Reps. Rahm Emanuel of Illinois and Chris Van Hollen of Maryland on the House side revived the only recently flagging fortunes of their party by thinking nationally, organizing locally and running rings around their congressional Republican counterparts in candidate recruitment, incumbent retention and fundraising.

But the Democratic gains also never could have happen if the Republican Party hadn’t been a co-conspirator to its own precipitous decline — something that would have been deemed impossible just four years ago, as President George W. Bush celebrated his narrow re-election by declaring a mandate for his policy agenda and his partisan allies in the congressional leadership talked about settling into a permanent majority status.

Public dissatifaction over the long U.S. military involvement in the Iraq war, anger over government mishandling of the disaster inflicted on the Gulf Coast by Hurricane Katrina in 2005, corruption scandals involving more than a handful of Republican lawmakers who had grown too comfortable in their seats of power, and a growing sense that a long period of economic growth was waning caused voters’ confidence in Bush and the GOP in general to plummet.

After big Democratic gains in the 2006 midterm elections deprived the Republicans of the dominance they had enjoyed in Congress for a dozen years, GOP officials and officeholders crossed their fingers and hoped for a better political atmosphere in which to run. But Bush’s continued deep unpopularity combined with an unraveling in the economy — capped by a full-scale crisis in the financial sector that erupted in the midst of the general election campaign — pancaked the Republicans’ hopes for a comeback.

The Democrats, while invigorated over their soaring political revival, need to be cautious about over-interpreted the mandate they have won. Polls taken throughout the 2008 campaign showed voters were more strongly expressing dissatisfaction with the Republicans than a warm embrace of the Democrats. From the Democrats’ sweep that accompanied the 1992 election of Bill Clinton as president to the backlash two years later that pushed the conservative “Republican revolution” to control of Congress just two years later to GOP’s presumptions of long-term control during the Bush era, the Democrats have many case histories of what to try to avoid.

The Republicans, though, face a rugged road to a comeback. McCain’s clear defeat — after a campaign in which he played up his image as a party “maverick” and scathingly criticized Bush’s performance as president — leaves the party with a leadership vacuum that will be likely not be filled without a fight. The party’s future direction will be determined by a tug-of-war between those who believe the Republicans must projected a purer conservative vision and those who say voters punished the party for playing only to its conservative base and ignoring the vast political center in which the nation’s elections typically are decided.

The decisions the parties make about their approach going forward, and their efforts to claim their stake on the nation’s electoral landscape for 2010 and beyond, will begin to be shaped even before the Democratic president and the expanded congressional majorities elected Tuesday take their oaths of office in January.