Barack Obama secured the presidency with the highest percentage of the national popular vote won by a Democrat since Lyndon Johnson in 1964. He also gained a wide margin of victory in the Electoral College. But his governing majority was secured in the contests for the Senate and House, where Democrats made significant advances. Indeed, the Congressional results confirm that the 2008 election was not just a personal triumph for Obama. It was a Democratic sweep of the sort rarely seen in recent decades.

The scope of the victory may not be known for weeks, as key Senate contests produced results so close that recounts could be required in as many as four states. In Minnesota, for instance, on the day after the election only a few hundred votes–out of 2.9 million–separated Democratic challenger Al Franken from Republican incumbent Norm Coleman in a contentious contest for Paul Wellstone’s old seat. At the least, Democrats will move from a narrow 51-to-49 Senate majority to a comfortable 56-to-44 advantage. That means they will no longer have to defer to Democrat-turned-independent Joe Lieberman, who backed John McCain for president. It also means that, although Obama is unlikely to enjoy a filibuster-proof majority of sixty Democratic senators, he will be able to use his bully pulpit to appeal to moderate Republicans–like Maine’s Susan Collins, who was re-elected on a promise to work across party lines–for support of needed reforms, a more responsible approach to foreign affairs and economic policies strong enough to meet the challenges posed by a deepening recession.

The new Senate will be marginally more progressive. New Mexico’s Tom Udall and his Colorado cousin Mark Udall, who as members of the House both voted against going to war in Iraq, against the Patriot Act and against most of the Bush administration’s misguided economic policies, will replace Republicans who were steady Bush backers. North Carolina’s Kay Hagan–who highlighted her differences with Republican incumbent Elizabeth Dole’s votes in favor of the administration’s free-trade agenda–will expand a loose-knit Senate fair-trade caucus that grew substantially when Democrats took control of the chamber two years ago. And New Hampshire’s Jeanne Shaheen, while hardly a raging liberal, is likely to be a solid ally for Obama, which John Sununu, the Republican senator she replaces, would never have been.

The same will be true of Virginia’s Mark Warner, the keynote speaker at the Democratic National Convention, who replaces relatively moderate Republican John “No Relation” Warner. On social issues in particular, the new Democratic senators, all pro-choice and sympathetic to gay rights, are aligned with Obama, and that should make it easier to win confirmation of his Supreme Court picks.

The Democrats who won Senate seats this year know they benefited from Obama’s appeal and his savvy campaign. Instead of running away from a presidential candidate who gave up early on Western and Southern states, challengers in those regions could count on Obama to keep the top of the ticket in contention and to boost turnout among young voters, as well as African-Americans in the South and Latinos in the West.

Obama was perhaps even more of a help to his party’s House candidates. Democrats who won seats in GOP-leaning districts in 2006, such as New Hampshire’s Carol Shea-Porter and Wisconsin’s Steve Kagen, held on to them with relative ease as the presidential nominee spent heavily on voter registration and turnout drives that aided down-ticket candidates. And Obama’s get-out-the-vote crusade surely helped progressives like New Mexico’s Martin Heinrich take Republican seats. The Democratic surge was not enough to win what could have been the sweetest Congressional victory: Michele Bachmann, a Minnesota Republican who launched into a McCarthyite rant on MSNBC’s Hardball, survived by a narrow margin. But Virginia Democrat Tom Perriello came within recount range of another crude conservative, Virgil Goode, who made headlines two years ago griping about the election of the first Muslim Congressman, Minnesota Democrat Keith Ellison.

Perhaps the best news is that many of the roughly two dozen new Democrats in the House arrive as experienced reformers, like former Common Cause director Chellie Pingree of Maine and economic populists like New York’s Dan Maffei and North Carolina’s Larry Kissell. Some, like Pingree, are likely to become leaders of an expanded Congressional Progressive Caucus. Others will work closely with the emerging bloc of fair-trade Democrats willing to break with the party’s House leadership to protect workers’ rights and the environment here and abroad.

House Democrats will have to prod Obama, as should their Senate peers. Certainly there will be a desire to aid the new president. But Obama and a more Democratic Congress arrive in Washington as part of a broad electoral rejection of regressive Republican rule. They are charged with a responsibility to change the way America is governed. Essential to that change must be a restoration of the system of checks and balances that was so diminished during eight years of the Bush/Cheney interregnum.