Even if the Democrats gain seats in both chambers of Congress in November, early forecasts suggest that any anti-Trump wave won't be particularly large compared to midterm elections of the past.

Why it matters: President Trump's low approval numbers and heightened enthusiasm among Democratic voters both point to Democratic success in November. But it probably won't be a wave of historic proportions, based on Cook Political Report's latest predictions. At best, it could allow them to win the House while barely shifting the Senate at all.

The back story: As the chart shows, both the House and Senate have nearly always moved against incumbent presidents in mid-term elections. See the big Republican swings against Barack Obama in 2010 and 2014 towards the top right, and, in the lower left quadrant, the Democratic swings against Dwight Eisenhower in 1958 and Gerald Ford in the post-Watergate midterm of 1974.

This year may be different. The Democrats will almost certainly make gains in the House — Cook Political Report predicts Democrats will gain between 25 and 40 seats, enough to win back control of the chamber.

• But the Senate math is tougher for them. Cook predicts Democrats will gain as many as two seats, which would give them control of both chambers, but they may also lose one.
• And there's no guarantee that the House wave will be strong. A CNN poll this week found that the Democrats' advantage over Republicans in the generic ballot has gotten weaker, and is now within the margin of error.

What to watch: If the Democrats gain seats in the House while the GOP gains in the Senate, it will be the first midterm in 36 years in which the two chambers of Congress moved in different directions.