The new CNN poll is good news for Republicans. Or is it?

Speaker of the House Paul Ryan, R-Wis., arrives for a meeting of fellow Republicans on the first morning of a government shutdown after a divided Senate rejected a funding measure, at the Capitol in Washington, Saturday, Jan. 20, 2018. (AP Photo/J. Scott Applewhite)

(CNN)At first glance, the new CNN poll is a bit of good news for Republicans looking to the coming 2018 midterms with trepidation.

On the generic ballot question -- if the election were held today, would you vote for a Democratic or Republican House candidate -- Democrats hold a 5-point edge among registered voters, down from an 18-point advantage in a CNN/SSRS poll conducted in December 2017.
Those gains on the generic ballot are due, primarily, to a double-digit jump between December and now in enthusiasm about the midterms among Republicans and Republican-leaning independents.
    That leap can be directly traced to the passage of President Donald Trump's tax cut plan at the end of last year and seems to justify the belief among GOP strategists that passing the tax bill was absolutely necessary to energize their base and give them a chance to avoid broad-scale losses in 2018.
    That semi-surge on the generic, coupled with Trump's modestly-improving approval rating (he's at 40% in the new CNN poll, up from 35% in December) might well relieve some of the dread that many party strategists have been feeling about the coming midterms.
    And there's no question that for a party which hasn't had a poll result to feel good about in quite some time, these latest numbers are reason for optimism.
    But there are other numbers in the poll that suggest that while the broad image of Trump and the Republican Party is improving, they still have a major base enthusiasm/intensity issue.
    Compare the two party bases on the enthusiasm question. Almost two-thirds (62%) of liberal Democrats say they are either "extremely" or "very" enthusiastic about the 2018 election; 46% of conservative Republicans feel the same way.
    When you ask the same generic ballot question among only those who are "extremely" or "very" excited about the 2018 election, Democrats hold a 56% to 41% lead. Among those who say they are less enthusiastic, Republicans have a 5-point edge on the generic ballot.
    Remember that two things are almost always true about midterm elections: 1) They are a referendum on the president and his party and b) there is a considerable drop-off of casual voters, meaning that the two party bases exert far more influence over outcomes than they do in presidential contests.
    Which makes the ongoing energy gap between the two parties' bases on the enthusiasm question concerning for Republicans. Sure, they are doing better among all voters but, if past is prologue, lots of these registered voters just won't show up to vote in the fall.
    These numbers land amid the first government shutdown since 2013, the anniversary of Trump's first year in office and women's marches all over the country on Saturday primarily focused on channeling liberal rage against Trump into organizing and turnout efforts for the election later this year.
    The tax bill clearly helped give the Republican base a boost of energy. But that could well be a blip on the radar if the party winds up bearing the brunt of the blame for this government shutdown, which is now entering its second day.
    What might you expect to see from congressional Republicans and Trump once the government re-opens? A series of bills and actions aimed specifically at the care and feeding of the Republican base. Those moves will be decried by Democrats and even many independents -- but they may be the only way for Republicans to survive the 2018 election with their congressional majorities intact.
    Without an energized base, a party is nowhere. Especially in a midterm election.
    CORRECTION: This analysis was updated to correct the name of the polling company that conducted CNN's December 2017 poll.