RICHMOND — Virginia's major-party nominees for governor engaged in some Twitter one-upsmanship Tuesday, challenging each other's positions on the coronavirus vaccines ahead of Thursday night's scheduled debate.

Democrat Terry McAuliffe set the stage Tuesday morning by releasing a plan for a “Virginia is for Vaccine Lovers” promotional campaign. Though full of specific policy proposals, such as using federal relief money to aim teams of public health workers at areas with low vaccination rates, the plan is framed by harsh criticism of Republican Glenn Youngkin’s opposition to any vaccine or mask mandates.

A few hours after the plan came out, Youngkin responded on Twitter with a political chess move, inviting McAuliffe to join him in recording a public service announcement encouraging vaccination.

“Hey @TerryMcAuliffe, I’m glad you’ve finally realized that a marketing campaign encouraging Virginians to get vaccinated against COVID is important. I released a PSA weeks ago. Where’s yours? Let’s put politics aside and film one together this Thursday at the debate — you in?” Youngkin tweeted.

With the delta variant sending cases soaring in Virginia to levels not seen since the first weeks of the year, primarily among the unvaccinated, the state has been scrambling to get more people to take the shots. About 65 percent of Virginians have received at least one dose of a vaccine.

Gov. Ralph Northam (D) has mandated masks for all K-12 school classrooms and has required vaccinations for state employees. McAuliffe has called for even wider use of vaccine mandates, writing in an opinion piece in The Washington Post last month that all health-care and school workers should be required to get vaccinated.

Youngkin has said vaccinations should never be required but should be a matter of personal choice, though he has encouraged everyone to get the shot. He released a video explaining his own decision to get vaccinated.

McAuliffe responded to Youngkin’s Twitter taunt with one of his own, dismissing the call for a joint PSA as a “gimmick” and suggesting another idea: “Join me in calling for vaccine requirements so we can end this virus once and for all. Step up Glenn.”

Youngkin replied: “Bipartisanship that will save lives isn’t a gimmick, Terry. Sad that you think that way.”

McAuliffe responded: “Glenn are you willing to call for all Virginians to follow the science and call for vaccine mandates except for medical and religious reasons, no anti-vaccine rhetoric involved? If so, I’m in. Let’s do it in Grundy,” referring to the site of Thursday’s debate.

By midafternoon, that’s where the exchange stood.

Even holding a debate has been a contentious issue for these two candidates. McAuliffe accepted invitations to five debates, while Youngkin had said he would agree to up to three. The Republican broke a 36-year tradition by refusing to participate in what’s usually the state’s premier gubernatorial showdown, the Virginia Bar Association debate, which had been set for July. Youngkin objected that moderator Judy Woodruff of PBS NewsHour had once donated $250 to the Clinton Foundation’s Haitian earthquake relief fund.

Last month, McAuliffe refused to participate in a debate at Liberty University after the Lynchburg school hosted an “election integrity” rally that Youngkin attended. McAuliffe said that event raised groundless questions about election security and undermined faith in democracy.

As it stands, the two are scheduled to meet only twice: Thursday night at the Appalachian School of Law in Grundy in Southwest Virginia, and at a Northern Virginia Chamber of Commerce debate set for Sept. 28 on the campus of Northern Virginia Community College in Alexandria.

Thursday’s debate is scheduled to start at 7 p.m. Susan Page of USA Today will be the moderator, with questions from political analyst Robert Holsworth and Richmond CBS6 news anchor Candace Burns.