“There’s been this pent-up opposition to lockdowns and mask mandates and so this is building on that,” said David Boaz, executive vice president of the Cato Institute, a libertarian think tank. “Now there’s this suggestion that if you don’t get a vaccine, you might not be able to do — we’re not quite sure what. I can see how there’s a market for that concern.”

The attacks also focus on an area that’s been a strength for Biden: his handling of the pandemic. Under Biden’s watch, vaccine distribution has significantly ramped up and, according to federal survey data, reports of vaccine hesitancy are decreasing. Covid-19 deaths have also plummeted from January highs, in part because larger portions of older Americans have been inoculated. But there’s been an uptick in infections in recent days as states have relaxed coronavirus restrictions.

Now the effort by some Republicans to create doubt about a vaccine passport program threatens to define the Biden administration effort while it’s still in the earliest phase, blunting its ability to roll out an idea that could be a popular project and putting the administration on the defensive.