Gallup isn't exactly the gold standard on polling any more, if they ever were (I always preferred Pew). Still, they have the advantage of a very long time series on a lot of relevant indicators. so they are worth paying attention to. Two new releases summarize some of their key data, with abundant tables and charts.
On midterm indicators and the GOP:
"Democrats need a net gain of 23 seats in the House of Representatives to win the majority this year, and since 1974, the president's party has lost an average of 22 seats in midterm elections. Weaker job approval ratings for the president and Congress suggest the seat losses this year could be greater than the average.
While the positive ratings of the economy could be a mitigating factor, the available evidence does not support this possibility. Economic ratings are less strongly related to midterm seat change than....other indicators, and typically when the economy is rated well, the president also has a high job approval rating. Although there are a limited number of comparable historical examples, the 2006 midterm is one in which Americans were relatively happy with the state of the economy but unhappy with the president, and Bush's GOP lost 30 seats in the House and their majority in that election."
On midterm turnout:
"While the proportion of Americans who vote in midterm elections is important for civic reasons, the difference in turnout between Republicans and Democrats holds potentially greater significance, as this can affect the outcome. On the eve of the 2018 midterms, all signs point to Democrats holding their own with Republicans on turnout.
This is notable because typical Republican advantages on voter turnout help the party overcome Democrats' usual advantage in party identification among the adult population, and therefore in voter support for Democratic candidates. Without that advantage, Republicans are far less likely to retain their majority in the U.S. House in this election."