Since President Obama issued his massive new budget, analysts have agreed that its combination of broad policy proposals and tax increases to finance them constitute a major gamble whose outcome will define his tenure.

But they're also a major challenge to a governmental system that has had great difficulty in recent years coping with less complicated, more incremental proposals on healthcare, energy and the other key underpinnings of Obama's effort.

"The odds are long" of being able to make such major changes in policy amid the economic crisis that has preoccupied lawmakers and made a bad federal fiscal situation far worse, Obama conceded last week.

Indeed, had he come to office with less ambitious goals, Obama might have been satisfied with pulling the nation out of its current difficulties.

But change was more than just a slogan for the Obama campaign. Part of his contention is that the extent of these problems requires more than a short-term fix.

In essence, the president and his advisers argue, the economy's systematic breakdown makes the broader changes on which he campaigned more necessary than ever. And Republicans are playing into the new administration's hands by defining themselves as naysayers trying to block changes the country wants and needs.

Both factors are helping Obama maintain backing from virtually all Democrats and a majority of independents, the very coalition that elected him in November.

Keeping that support will be vital in helping him overcome the institutional roadblocks he faces in Congress, especially when the time comes to enact the healthcare and energy proposals for which his budget provides little more than a first step.

That will be far harder than winning initial approval for his budget, given a solid Democratic majority in the House and the fact that normal Senate filibuster rules don't apply to budget resolutions. That means a budget can pass with only Democratic votes, though that may require some compromise with the more conservative Senate Democrats.

Passage of the complex legislation to flesh out the proposals, Obama observed Monday, will require "difficult tradeoffs." He'll need to compromise more on the details than he was willing to do on the initial stimulus package.

One encouraging note is that bipartisan healthcare talks are already under way in the Senate, since some Republican support will be necessary to pass any legislation.

Meanwhile, the initial budget fallout has shown Obama's political challenge. It's easy for critics to make headline-grabbing arguments that condemn his proposals as a massive increase in government power that will raise the taxes and saddle future generations with a huge debt.

But the GOP critics hardly come to the argument with clean hands, given the spending excesses of the Bush years. Most initial polling shows support for Obama's budget approach, though a Rasmussen Research sampling indicates he has not yet convinced Americans it's necessary to tackle healthcare reform before the economy recovers.

And while there is some truth in the substance of Republican criticism, the GOP's main hope is to drive down Obama's public approval ratings so more conservative Democratic lawmakers will be more hesitant to approve his proposals.

Even if Republicans fail in the short term, they hope to succeed by the 2010 mid-term elections, especially since economic recovery may well be slower than the administration projects.

But they may need to do more to dent Obama's political advantage. While the GOP probably will regain some lost House seats next year, Democrats almost certainly will keep a working majority. And the Democrats could gain Senate seats and reach the 60 needed to curb GOP filibusters.

Fearful of being defined by their negativism, Senate Republicans now talk of the need to develop viable alternatives to Obama's proposals. That seems wise in a time of widespread agreement on the need to deal with the problems he is confronting — an expensive, inefficient healthcare system and an over-reliance on oil, especially oil from abroad.

In the end, Obama's program may prove more than a traditionally inefficient Congress can enact. But he's likely to get political points for trying. And achieving even a down payment on his ambitious plans would mark significant progress over the gridlock of recent years.

Carl P. Leubsdorf is a former Washington bureau chief of the Dallas Morning News. E-mail him at carl.p.leubsdorf@gmail.com.