Amy Walter of the respected Cook Political Report asks the question: "If Democrats Are Doing so Great, Why Don't They Have a Bigger Lead on Generic Ballot?"

Indeed, the Democrats' lead in the generic ballot has gone from double-digits last year to less than 7 points, according to several polls.  The reason may surprise you:

Here's my best guess.  First, we tend to spend too much time looking at the margin instead of the vote itself.  For example, the Quinnipiac poll in March had Democrats up 10 points.  In April, that lead was down to just 3 points.  The headline: Democrats lose their lead!  But, let's take a closer look at what actually changed between March and April.  In March, 48 percent said they'd like to see Democrats win control of Congress to just 38 percent who said they'd want Republicans in control. In April, 46 percent wanted to see Democrats in control (a slight 2 point drop), while 43 percent picked the GOP (a more impressive 8-point improvement).

What does this mean?  It means that Republicans are "coming home."  Even in a terrible year for the GOP, they are not going to perform much worse in the national vote than 43-44 percent.  In 2006, for example, Republicans took 44 percent of the national House vote, even as many polls leading up to Election Day showed Republicans in the high-30's.  In 2008, an even more politically horrific year for the GOP, Republicans garnered 43 percent of the national House vote.  In both cases, Republican voters, many reluctantly, "came home" to the GOP in the end.  What's happening now is that these voters are coming home sooner.  Given our intense polarization, and a president and a news media that fans those partisan flames, this shouldn't be all that surprising.

As disgusted as many GOP voters have been with the party, their voters are still far more likely to pull the lever for Republicans rather than the Democrats.

And the Democrats are making it ridiculously easy for them.

In some of the most competitive races around the country, Democrats are insisting on nominating "pure" progressives – clones of Bernie Sanders and Elizabeth Warren.  Younger party members may be cheering the radicals on, but given that young voters historically don't vote in off-year elections, unless there are some huge surprises, most of the far left radicals will lose.

This gives Republicans a chance to hang on to the House.  But other factors must break their way for that to happen.

The question for the fall, of course, is where those who currently put themselves in the "undecided" category break.  We know these voters are much less engaged in politics.  They are less attached to party and partisanship.  We lump this group into the category called "independents."  And, here's what we know about them: they don't like Trump.  Overall, about one-third to 40 percent of self-described independent voters approve of the job Trump is doing as president.  And, as we know, how you feel about the president is correlated very closely to how you vote in a mid-term election.

In the latest Marist/NPR/PBS poll (April 10-13), for example, Trump's job approval rating among independents is 38 percent.  On the generic ballot question in that same poll, the congressional Republican gets 32 percent of the independent vote.  A late April Quinnipiac poll showed Trump with a 33 percent job approval among independents, and 36 percent of independents say they will vote for a Republican in the fall.

In the end, the fate of the GOP in the midterms will be determined as it always has been: turnout and approval of the president.  Democrats know this, which is why they are trying to shape the narrative that makes a Democratic victory inevitable.  A lot will depend on the Republican get-out-the vote effort.  Perhaps even more will depend on Democrats' ability to sideline their radicals and nominate more moderate candidates.  The fact is, any talk of a "blue wave" is bogus at this point.