MIDLOTHIAN, Va. — For Democrats in Virginia, the legend of last year’s general election, when the party romped statewide, has been burnished to a golden glow. If there is a blue wave nationally in the midterms, many will look back to Virginia in 2017 as an early sign.

But the state is going to the polls again for primaries on Tuesday, at a moment when some analysts suggest that the wave could be flattening to a ripple. President Trump’s job approval rating has ticked up and the unemployment rate is the lowest in nearly two decades.

Whether Democrats’ zeal to rebuke Mr. Trump remains strong will be closely scrutinized as Virginians vote in contested primaries for nine of the state’s 11 congressional seats. The Center for Politics at the University of Virginia calls it the busiest federal Primary Day in the state’s modern history.

The results will be sifted for hints of what to expect in this state and across the country in November, when Democrats hope to win control of the House of Representatives as a check on what they see as an increasingly reckless White House.