Washington ? It was fascinating to watch the three top contenders for the Democratic nomination discuss their concept of the presidency during Tuesday night’s MSNBC debate in Las Vegas. But it was also stunning to realize that the three current and former senators who have survived the shakeout process – Hillary Clinton, Barack Obama and John Edwards – have not a day of chief executive experience behind them.

By contrast, the Republican field is loaded with people who are accustomed to being in charge of large organizations. Mitt Romney and Mike Huckabee were governors of their home states of Massachusetts and Arkansas, Rudy Giuliani served as the mayor of New York City, and John McCain, as he likes to remind audiences, commanded the largest squadron in the Navy air wing.

In the past, voters have preferred to entrust the White House to those with executive credentials. John Kennedy was the last sitting senator to be elevated into the presidency. Since then, the former governors of Georgia, California, Arkansas and Texas have dominated the list of successful candidates.

All of them stumbled during their tenures in the White House, and only Ronald Reagan left the presidency with his place in the history books seemingly securely enhanced.

But the public remains convinced that the Oval Office is a place for executive talents – which makes the current Democratic field something of an anomaly.

Romney’s comeback victory in Michigan on Tuesday reflects that bias. He began to regain his footing after Iowa, when he subordinated his ideological claim to being the conservative champion in favor of portraying himself as a tough-minded executive who could reform both laggard private businesses and swollen, ineffective government bureaucracies.

He drew a useful contrast to “broken” Washington, the home base of Sen. McCain and two of his three Democratic colleagues – Obama and Clinton. Edwards is a former senator.

Huckabee had made a similar case for himself, citing his decade of leadership in Arkansas. And Giuliani had asserted a record of accomplishment in rescuing New York from fiscal crisis and reducing the city’s crime and welfare rates.

All of this places an unusually heavy burden on the three Senate Democrats to show they can do more than talk a good game of leadership – and actually lead.

What emerged in the discussion that Brian Williams and Tim Russert encouraged in Las Vegas were three very different concepts of presidential roles.

Obama, who answered first, said it involved much more than managing the office and seeing that “the paperwork is being shuffled effectively.” He said it centers on setting national goals, recruiting people of different perspectives and then mobilizing public support behind their policies. He acknowledged a degree of disorganization in his personal and business life and said he needs help “keeping track” of things.

Edwards, as is his habit, described himself as a battler – ready to fight passionately for his causes, and acknowledged that emotion sometimes clouds his judgment.

Clinton called herself a change agent, with 35 years of experience, but emphasized that the presidency involves managing the bureaucracy and insisting on accountability – not just setting goals. A perfectionist, she acknowledged that impatience for results is at times a problem for her.

She faulted President Bush as much for his mismanagement of recovery efforts after Hurricane Katrina and other problems as for his policy judgments, prompting Obama to counter with the argument that it was a lack of vision and candor, not simply of management, that made Bush a failure.

The discussion provided more clues to the differences among the three Democrats than voters had previously been given. In concept, Clinton’s definition of the office is more complete than either of the others’, probably reflecting her own close-up view of the presidency during the eight years she lived in the White House.

But the very failings she and Obama acknowledged earlier in the debate, when apologizing for the words and actions of their supporters that had inflamed racial tensions in the campaign, showed the difference between discussing leadership and practicing it.

The burden of proof of readiness to be president is heaviest on those who have never borne executive responsibility. And that is something voters will have to weigh, whichever of the Democrats is the nominee.