Republicans have taken a clear lead over the last month on the question of whether likely voters favor a Republican-controlled Congress or one dominated by Democrats. Some polls using this generic question show the Republicans ahead by as many as 7 points among likely voters, a tally rivaling the polls immediately before the Republican landslide in 2010.

Yet Republicans have made few, if any, polling gains in their fight for control of the Senate. Their position might have even deteriorated. They need to win six seats held by Democrats. Michigan and North Carolina have drifted toward the Democrats. Kansas is newly competitive. Colorado and Iowa, two presidential battlegrounds, remain extremely close. And although the Republicans have consolidated their advantage in Louisiana and Arkansas, Mary Landrieu and Mark Pryor still seem doggedly competitive in states where President Obama lost by 23 percentage points.

The term “wave election" is not formally defined, but the current Senate polling picture bears little or no resemblance to the outcomes of the 2006 or 2010 midterm elections. Now those were waves. The party opposed to the president swept almost every competitive open contest; routed almost every incumbent of the president’s party in a state won by the opposition in the prior presidential election; and even defeated incumbents in states that voted for the president. None of these conditions appear true today.

How are the Democrats holding up so well, even while the polls show Republicans faring well nationally? Here are four possibilities, in no particular order:

There isn’t a wave, just a turnout gap

The large Republican advantage on the generic ballot may not reflect a wave, just a large Democratic turnout problem.