There is a widespread assumption that President Obama has expanded the electorate and inspired booming voter turnout. One could make a case for that based on the 2008 election. But since then, not so much.

Looking back over the past 15 years, the biggest surge in voter turnout came during George W. Bush's presidency. In the Obama years, turnout actually declined in both the 2012 presidential and the 2014 congressional elections.

In 2000, about 105 million Americans voted for president. In 2004, 122 million did. That's a 16 percent rise, the largest between two presidential elections since 1948 and 1952. Turnout increased further in 2008, to 131 million. That's a 7 percent increase over 2004.

There's a similar pattern between the off-year elections during that period. Turnout was 66 million in 1998, when Republicans were mulling the impeachment of Bill Clinton. It increased to 73 million in 2002, when Bush's post-9/11 job approval remained high, and to 80 million in 2006, when Bush's job approval was languishing at levels similar to Obama's this year.

One conclusion here is that increased turnout can result from both hearty approval and vitriolic opposition. You see both in the numbers: Bush won 11.6 million more votes in 2004 than he did in 2000, but John Kerry received 8 million more votes than Al Gore. Republicans apparently did a better job than Democrats of getting their votes out that year, but feeling was high on both sides.

Four years later, Democrats did a much better job turning out the vote. The Obama campaign inspired many young voters and blacks to join the electorate. It targeted four previously safe Republican states -- Virginia, North Carolina, Indiana and Missouri -- and carried three of them, losing Missouri by an eyelash. North Carolina, though not the fastest-growing state, had the biggest percentage increase in turnout between 2004 and 2008 as the Obama campaign registered thousands of blacks and students.

Those numbers showed enthusiasm for Obama. But the numbers since then have not. Off-year turnout spiked from 80 million in 2006 to 86.5 million in 2010. But Democrats got 3.4 million fewer votes than they did four years earlier, and Republicans got 9 million more.

The 2012 Obama turnout operation outshone the Republicans, and Obama was re-elected. Even so, he got 3.6 million fewer votes than he did four years before. That proved to be enough, because Mitt Romney received only 1 million more votes than John McCain. Overall turnout sagged from 131 million to 129 million. Both sides seemed dispirited.

That year turnout was up, but only by 1 percent, in the 10 states that both parties targeted. Turnout fell 1.9 percent in the 23 safe Republican states and 3.4 percent in the 18 safe Democratic states. Republican votes rose in all three categories if you leave out New York and New Jersey, where Hurricane Sandy depressed turnout. Obama votes were down 8 percent in Republican states and 5 percent in Democratic states, but only 2 percent in target states.

Obama's victory, like Bush's in 2004, owed something to superior organization. But unlike Bush's win, Obama's looked less like a measure of enthusiasm than a grudging acquiescence to the status quo.

That's not what the 2014 off-year results look like. In the 20 states with seriously contested races for Senate or governor, turnout (as measured by total vote for the House) was up over 2010, but by just 1 percent. In the 30 states without such contests, it was down 18 percent --15 percent in safe Republican states, 20 percent in 2012 target and safe Democratic states.

Tentative conclusions:

Organization matters. But superior Democratic organization can only marginally shift the results. It was enough for Obama in 2012 but not enough for Democrats in 2014. The enthusiasm for Obama that propelled turnout increases in 2008 has evaporated. Turnout slumped in Democratic states -- especially California and New York -- except where serious statewide contests and attendant organization brought Democrats out. Even in states where that was the case, Democratic turnout declined while Republican turnout increased. Americans are increasingly tactical non-voters. In target states or in states with serious statewide contests, they'll vote. In other states, many don't bother. Republicans may well benefit again from anti-incumbent feeling in 2016, as Obama did in 2008. Democrats need someone who can arouse the enthusiasm Obama tapped in 2008, which has since declined.

Can Hillary Clinton fill that bill?