Over the past 10 weeks, Democrats have won three critical elections that could presage more successes this fall, and that also expose the pitfalls posed by factions trying to push the party to the left. 

This week's Democratic victory by Patty Schachtner in a special Wisconsin state Senate race has roiled Republicans and excited Democrats nationally; she won by 11 percentage points in a Republican district that President Donald Trump carried by 17 points in 2016. It followed the first Democratic victory in a U.S. Senate race in Alabama in 25 years and decisive wins for governor and other contests in Virginia.

There have been dozens of better-than-expected Democratic showings in local contests around the country during the first year of the Trump presidency. This suggests that there's an energized Democratic base and a divided Republican one. The victories offer Democrats a guide to success in the midterm congressional elections and statehouse races this fall: Pick off a handful of seats in conservative districts in the South and Southwest and score big in swing states like Wisconsin and Virginia.

Most of the winners this year have been mainstream Democrats, progressive by the standards of their local districts but resistant to the arguments pushed by the Sen. Bernie Sanders faction for single-payer health insurance, impeachment of Trump and free college tuition.

The Wisconsin result set off alarms from the state's two most prominent Republicans, U.S. House Speaker Paul Ryan and Gov. Scott Walker. Schachtner campaigned as a "common-sense" Democrat who vowed to work with Republicans. She didn't attack Trump by name, but decried the "negative" tone in politics and declared, "We need to be considerate and help people when they're down."

The Democrats' other prominent winners also resisted the call of their party's left wing. In conservative Alabama, Doug Jones promised a bipartisan alternative to the hyperpartisan Roy Moore, the ex-judge accused of sexually abusing underage girls. Virginia's new governor, Ralph Northam, repelled pressure to call for impeaching Trump and rejected the idea of replacing the Affordable Care Act with a government-financed national insurance plan. He is a pediatric neurologist.

Instead, these candidates emphasized other progressive bona fides. Jones campaigned aggressively for black votes as the former federal prosecutor who convicted the Ku Klux Klansmen who killed African-American girls in the 1963 Birmingham church bombing. Northam assailed Republican health-care and tax plans and pushed for expanding Medicaid in his state. Schachtner, a county medical examiner who grew up on a Wisconsin farm, stressed her environmentalist credentials in contrast to her Republican opponent, state Representative Ralph Jarchow, and, by extension, Walker.

Democrats could undercut their prospects for a good year if activists successfully set up ideological litmus tests in divisive primaries that might produce more controversial nominees. By contrast, the victories in Virginia, Alabama and Wisconsin offer the more promising strategic blueprint for Democratic candidates: calm, progressive centrism that signals relief from Trump's harsh extremism.

That's the right formula for a blue wave.

Albert R. Hunt is a Bloomberg View columnist, before which he was a reporter, bureau chief and executive Washington editor at the Wall Street Journal.