Terry McAuliffe, the Democrat running for governor of Virginia, distilled the election into a single sentence.

“It all adds up to the same thing here: Donald Trump, Donald Trump, Donald Trump,” he said the other day.

Contests for governor in Virginia have long been a barometer of the national political mood a year into a new presidency. For Democrats, the stakes have never seemed higher: A defeat for Mr. McAuliffe, a popular former governor seeking his old job back, could deal a devastating blow to the party’s confidence heading into next year’s midterms and to its strategy of running against Mr. Trump even when he is not on the ballot.

For Republicans the stakes are less fraught: Their nominee, Glenn Youngkin, a first-time candidate, could lose narrowly given Virginia’s increasingly blue tinge but still represent a proof of concept that a G.O.P. candidate can unite the party’s moderates and hard-liners without going all in on Trumpism.

Whether it is Mr. McAuliffe hammering away at Mr. Trump’s attempts to subvert the 2020 election or Mr. Youngkin walking a Trump tightrope — nodding to the base on election fraud, while keeping the former president partly at arm’s length — Mr. Trump has been an unavoidable factor in the Virginia campaign.

The unexpectedly close contest, which is effectively the opening act of the 2022 midterms, will also test the two parties’ appeal to the most crucial and coveted voters nationwide — those in populous and diverse suburbs, who are widely expected to decide the Virginia race as well as control of Congress next year.

“I think every Democrat is following Virginia as a bellwether,” Gordon Hintz, the Democratic leader of Wisconsin’s State Assembly, said. “It definitely set the tone in 2017 for the 2018 cycle.”

Beyond the broad-brush strategies, each candidate has landed on a favorite issue in the final two weeks before the Nov. 2 election, both of which are likely to feature prominently in races elsewhere. For Mr. McAuliffe, the issue is abortion rights, newly under threat in the Supreme Court. For Mr. Youngkin, the issue is parental control of schools, which could broaden his appeal to independents who abandoned the G.O.P. under Mr. Trump.

Polls show a statistically tied race in Virginia, with worrying implications for President Biden, who easily won the state. Democrats say they are battling stiff but temporary headwinds: rising inflation, the lingering pandemic and an impression of Democratic incompetency in Washington, where the party has been in a stalemate over passing its big domestic priorities.

“Youngkin, to his credit, has done a real good job of maintaining the loyalty of the Trump base while attempting to generate some suburban defections from the Democratic Party,’’ said Bob Holsworth, a longtime Virginia political analyst. “If a Republican can win in Virginia talking about critical race theory, about his pro-life beliefs — a state Biden carried by 10 points — it would be far more than a wake-up call for Democrats. It would be somebody playing reveille in their bedrooms with a trumpet.”

Virginians, who vote for governor a year after presidential elections, have a long record of rebuking the party that holds the White House. Mr. McAuliffe’s win in 2013, a year after President Barack Obama was re-elected, was the sole exception in four decades. During the Trump years, the state swung even more toward Democrats in state and federal elections, driven by college-educated voters in the suburbs of Northern Virginia and Richmond who rejected the president’s divisive leadership.

Mr. Biden’s capture of 54 percent of suburban voters nationally last year was chiefly what put him in the White House. Suburbanites tipped battleground states including Pennsylvania, Georgia and Arizona. They also hold the key to the majority of competitive House races in 2022. Whether Democrats have earned suburbanites’ long-term allegiance or Mr. Biden merely “rented” them, as strategists like to put it, is a major question that the Virginia election could help clarify.

Republicans think they already know the answer. “The closeness of this race suggests the suburban swing voter is moving back to Republicans fast,’’ said Dan Conston, president of the Congressional Leadership Fund, a Republican super PAC that focuses on House races. “That is a warning sign for the many incumbent Democrats in swing suburban districts.’’

But Democrats believe that fear of Trumpism will keep the suburbs in their corner. Representative Sean Patrick Maloney of New York, chairman of the Democrats’ 2022 congressional campaign arm, said recently he was advising members in competitive suburban seats to run against “Trump toxicity without Trump on the ballot.”

“You’ve got to remind them the other side is for insurrection, when we’re trying to do infrastructure,” Mr. Maloney said, speaking to the liberal podcast “Pod Save America.” “They’re for fighting, when we’re trying to fix problems.’’

From the beginning, Mr. McAuliffe’s playbook has been to fuse Mr. Youngkin with Mr. Trump in voters’ minds. A new TV ad this week tries to link Mr. Youngkin to the former president’s equivocation about the white supremacists who marched in Charlottesville in 2017.

Mr. McAuliffe was handed fresh ammunition last week when Mr. Trump phoned in an endorsement of Mr. Youngkin to a rally that began by reciting the Pledge of Allegiance using a flag that organizers said had been carried on Jan. 6 in Washington. Mr. McAuliffe pounced, and Mr. Youngkin, who had not attended the rally, issued a statement calling the use of the flag “weird and wrong.”

Mr. Youngkin has tried to straddle the party’s divisions, appealing to Mr. Trump’s devotees as well as to moderate Republicans and independents. The enthusiasm edge that some polls show Virginia Republicans hold over Democrats suggests he has had some success in uniting the party.

That’s not an easy feat. “Youngkin seems more adept at trying to avoid Trump,’’ said Anna Greenberg, a Democratic pollster who is working for several Senate candidates in competitive 2022 races. “The degree to which that is successful will be a strong signal to lots of races around the country.”

Mr. Youngkin began the general election emphasizing the conventional Republican issues of taxes and job creation, but he is now aggressively leaning into conservative attacks on the way race is taught in schools and on giving parents more control.

A yearlong uproar in Loudoun County, targeting school board members over policies about racial equity and transgender students, suggests that Mr. Youngkin may be able to harness an issue that not only turns out conservatives, but persuades some suburban moderates.

Jon Seaton, a Republican strategist from Virginia, said the schools issue was breaking through to suburban parents. “In my little focus group on the sidelines of soccer games on weekends — I’m fairly certain they didn’t vote for Trump in 2020 — at least some are extremely frustrated by what’s going on in the public schools,’’ said Mr. Seaton, who consults for candidates around the country. “It’s certainly possible that education, for the first time in a very long time, becomes something that Republican candidates run on.’’

Pressing the issue, Mr. Youngkin has spent more than $1 million on a TV ad that plucks a statement of Mr. McAuliffe’s from a debate slightly out of context, in which he said, “I don’t think parents should be telling schools what they should teach.”

A Fox News poll of likely Virginia voters conducted last week showed a split decision on education. By a 23-point margin, parents among likely voters said they should have a say in what schools teach. However, when asked which candidate they backed, parents preferred Mr. McAuliffe 53 to 43 percent.

For Mr. McAuliffe’s part, abortion is the issue he has leaned into in the race’s final stretch, spending heavily on a TV ad showing hidden-camera video of Mr. Youngkin acknowledging that he must publicly downplay his opposition to abortion to win independent voters, but promising to go “on offense” if elected.

A second McAuliffe TV ad on abortion predicted that the Supreme Court would overturn Roe v. Wade and featured Mr. Youngkin saying he opposed adding a right to an abortion to Virginia’s constitution.

Historically, a single-minded focus on abortion has driven mostly conservative voters. Now that abortion opponents appear on the brink of achieving what they have long sought, the power of the issue may shift toward Democrats. Its ability to motivate voters is receiving a trial run in Virginia.