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The bidding war that has defined the Democratic presidential race reached its apogee of absurdity earlier this month when Bernie Sanders had to explain that, no, he had no plans to erase voters' credit card bills.

Questioned about his proposal to wipe away $81 billion in personal medical debt in a New Hampshire interview, the Vermont socialist told the Concord Monitor and "I don't believe we wipe out credit card debt. You want to buy a yacht, and you go in debt, hey, that's your decision."

It is easy to understand the pent-up demands of Democratic voters. In this century, the only time that the Democrats have controlled both the White House and Congress was during the first two years of Barack Obama's presidency.

While passing the Affordable Care Act by the narrowest of margins in 2010 remains Obama's most enduring legacy, it also played a major role in dethroning Nancy Pelosi as speaker in that year's midterm elections. As a result, Obama's powers were mostly limited to foreign policy and executive orders during the final six years of his presidency.

Thwarted in their dreams, the 2020 Democrats — exemplified by Elizabeth Warren — have put together more plans than the Allies drew up in preparation for D-Day.

Whether it's Warren's $2.75 trillion wealth tax, Sanders' $16.3 trillion Green New Deal, Pete Buttigieg's proposal to expand the size of the Supreme Court or Kamala Harris' belief that she could, if necessary, enact far-reaching gun-control measures through executive orders, the Democratic presidential candidates do not lack for ambition.

And that is in addition to the increasingly high-decibel but hard-to-follow disputes that have punctuated every debate over whether to end private health insurance under "Medicare for All" plans.

The problem with all this is not the aspiration to patch the major gaps in our health care system, confront gun violence, or deal with the glaring inequities in the you-win-I-lose economy. Rather, it is the inescapable reality that — barring a sea change in American politics — most of these goals are unlikely to be fully achieved in the coming decade.

If a Democrat wins the White House in 2020, the biggest job facing him or her would be cleaning up the muck from four years of Donald Trump's presidency. Repairing the willful damage to national institutions (from the FBI to the weather bureau) will only be matched by the desperate need to reassure the world that America is again a loyal ally and a beacon of hope for those battling despotism.

On election night 2020, congressional Democrats would be trading high-fives if they won the necessary three or four (in case Doug Jones loses his re-election bid in Alabama) seats needed to create a 50-50 Senate. Under that optimistic scenario, the new Democratic vice president could never stray far from Capitol Hill because the VP's vote would constantly be needed to break party-line ties.

With the Democrats' working majority dependent on centrists like West Virginia's Joe Manchin, there would be no appetite for vast legislative restructuring of the economy, energy consumption or health care. The best that a new Democratic president could hope for in the short run would be incremental changes.

Remember: Trying to do too much too fast caused not only Obama, but also Bill Clinton, to lose control of the House after two years in the Oval Office.

As the last two Democratic presidents have belatedly learned, governing with executive orders is like building sand castles on the beach in the face of the incoming tide. What seems like crowning accomplishments will be quickly destroyed by the next GOP wave.

Two-term Colorado Sen. Michael Bennet captured something important about the realities facing a Democratic president in 2021 when he repeatedly said and tweeted, "If you elect me president, I promise you won't have to think about me for two weeks at a time. I'll do my job watching out for North Korea and ending this trade war. So you can raise your kids and live your lives."

(Sadly, because the DNC's overly rigid rules cannot distinguish between serious presidential contenders and gadflies like Andrew Yang, Bennet was barred from the September debate and is unlikely to make the cut for October.)

Bennet, as daunting as his long-shot dreams currently appear, understands that a Democratic presidential victory will be partly powered by voter fatigue from Trump's antics, bile, deceit and blowhard incompetence. A soothing figure in the White House, whose fragile ego is not on hourly display, would probably represent welcome relief for the majority of voters who are not compulsively watching cable news.

The problem right now is that most of the leading Democrats (even Joe Biden) are running on ambitious legislative agendas that would offer high drama on Capitol Hill in 2021 but little chance of short-term legislative victory.

Too many pundits have been publicly brooding that the Democratic candidates have already veered too far to the left to win in 2020. At a time when the National Republican Congressional Committee (the political arm of the House GOP) uses the word "socialist" more often in its news releases than the word "the," aggressive moderation would not necessarily protect the Democrats from exaggerated Republican attacks.

Rather, the potential difficulty is more complex. The 2020 Democrats — in their efforts to inspire activists and wrest the nomination — are unrealistically raising expectations of what can be accomplished in office.

An abrupt reckoning with reality in 2021 would further undermine trust in democracy and, yes, the Democratic Party. Maybe it's time for Warren, Sanders and the others to begin to admit that their plans — no matter how detailed — are more aspirational than legislative blueprints.

Walter Shapiro has covered the last 10 presidential campaigns. He is also a fellow at the Brennan Center for Justice at New York University and a lecturer in political science at Yale. Follow him on Twitter @MrWalterShapiro.