President Obama’s European tour was a triumph. Among other accomplishments, he and British Prime Minister Gordon Brown managed to secure a commitment from G-20 nations to provide $1.1 trillion to less-developed countries, not just to help their battered economies, but to increase global demand. The Financial Times observed that few doubted Obama’s success except for his “conservative critics” at home. (Since so many of them live in an alternate universe, they have difficulty grappling with the realities of this one.)

Now Obama comes home—and here is the hard part. Will Democrats in Congress believe enough in his broad agenda of change to enact it? On health care, which Republicans desperately want to postpone on the calculation that delay presages defeat, the viable, perhaps essential, course is to include it in the budget reconciliation bill. The reason? Reconciliation requires only 50 votes and Vice President Biden to break a tie in the Senate. Under that scenario, health reform could be stopped only if Democrats break ranks.

The Republicans, despite their hypocritical moaning about the process now, showed in the Reagan and Bush years that a serious governing party has to be ready to invoke reconciliation to legislate its priorities. Ronald Reagan achieved a massive redistribution of wealth upwards and a partial shredding of the social safety net with the aid of a fiercely united party and a brace of Democratic defections. For all his domestic fecklessness, the first George Bush secured the Supreme Court confirmation of the discredited and ideologically rigid Clarence Thomas on an almost entirely party-line vote. The second Bush amplified Reagan’s approach, with congressional Republicans marching in weary lock-step almost to the bitter end.

Democrats, in contrast, haven’t been an effective governing party since the mid-1960s—except for a brief NAFTA-passing, deficit-reducing moment at the dawn of the Clinton years. There have been big progressive ideas pushed through Congress; Ted Kennedy has achieved more than many presidents. But the hallmark achievement of the Carter administration—the Camp David Accords between Israel and Egypt—was almost entirely a product of presidential prerogative. Carter’s signature initiative on energy failed—partly due to his delay in proposing it and then because of Democratic dissent in Congress. Clinton’s health-care bill came to grief in exactly the same way.

In the past, Democrats were disunited because so many of them were Southern conservatives or self-styled “moderates.” The former are mostly gone; it is the latter who now hold the balance of power. This is where Barack Obama’s strategy of expanding the electoral playing field may pay decisive dividends. Indiana Sen. Evan Bayh, who voted against the Obama budget, may ignore the fact that the president carried his state. But other moderates appear to be coming around to the idea of enacting a budget reconciliation that includes health reform.

After health reform, the test of Democratic governing capacity will move on to energy and other tough challenges, including the possible need for a new round of financial bailouts. (I know the White House doesn’t like that word, but that’s how opportunistically populist Republicans will cast it.) These legislative initiatives will require filibuster-proof majorities of 60 votes. The Democrats will have 59 votes in the Senate once the GOP, running out of increasingly far-fetched legal shenanigans, can no longer disenfranchise the state of Minnesota by delaying Al Franken’s seating in the Senate. If Senate Democrats then forge a united front around proposals that largely reflect the Obama program, they will need just one Republican vote to govern.

If there are too many Democratic defectors or too few Republican crossovers, the Democratic reign as a governing party may be short-lived. The mid-term defeat of 1994 could be repeated (though that might also require that the “green shoots” of economic recovery, which White House economic advisor Larry Summers says he has spied, fail to materialize.)

So this year—with this president at the height of his popularity and the economy in the depths of crisis—marks the critical test. If Obama could keep the tantrum-ready French President Nicolas Sarkozy from walking out of the G-20 summit, then don’t bet that he can’t keep defection-prone Democrats in Congress from walking away from historic legislation. At stake is not just a few laws but the future relevance of the Democratic Party and the prospect of a new progressive era for America.