New guidance from the Centers for Disease Control and Prevention (CDC) is throwing Congress into flux and sparking tensions over a central question: to mask or not to mask?

Some lawmakers, like Sen. Tim KaineTimothy (Tim) Michael KainePelosi calls for diplomatic boycott of Beijing GamesThe Hill’s Morning Report – Presented by Facebook – Biden wants Congress to pass abortion bill, pushes for Mideast cease-fireThe Hill’s Morning Report – Presented by Facebook – Israel-Hamas carnage worsens; Dems face SALT dilemmaMORE (D-Va.), are slowly adjusting.

Kaine was wearing a mask as he exited a committee hearing on Tuesday. But roughly an hour later, approaching the Senate basement, he was maskless. Passing by the same spot minutes later, he once again was masked.

Im in a transitional phase. … What I often do is if Im with somebody who is wearing a mask, Ill put one on too, Kaine said, asked how hes deciding when to wear a mask. But Im transitioning in my own head.

Individual decisionmaking is complicated by different rules that have created parallel universes in Congress.

On one side of the Capitol, senators are largely shedding their masks. On the other, masks are still required on the House floor, unless speaking.

Since Ive been vaccinated, Im following CDC guidance, said a maskless Sen. John CornynJohn CornynVoting rights hit wall in Congress amid GOP overhaulsGOP split on counteroffer to Biden’s spendingPolice reform talks hit familiar stumbling blockMORE (Texas), a member of GOP leadership. I think weve all learned a little bit about how to stay a little healthier. … I just think a little common sense goes a long way.

The two chambers have had their own cultures on how to handle the coronavirus pandemic.

All but a handful of senators have disclosed that they are fully vaccinated, making it easier for senators to inch back toward normalcy. But while all House Democrats have been vaccinated, a CNN survey found that only 95 of the 211 GOP House lawmakers have confirmed they are vaccinated.

The House is keeping its mask mandate for the floor, unless a member is speaking, and in committees. Speaker Nancy PelosiNancy PelosiThree GOP lawmakers fined 0 for flouting House floor mask rulesProblem Solvers Caucus backs Jan. 6 commissionGOP splits open over Jan. 6 commission voteMORE (D-Calif.) also disclosed Democrats were extending proxy voting until early July.

Those decisions are sparking grumbling from House Republicans.

It is time we follow the science, lead by example, and fully return to work to serve the American people. … Ineffective remote procedures have hindered congressional operations for too long and should not continue for the duration of the 117th Congress, dozens of House Republicans, including Minority Leader Kevin McCarthyKevin McCarthyTrump calls for Jan. 6 commission debate to end ‘immediately’Problem Solvers Caucus backs Jan. 6 commissionScalise urges House Republicans to vote against Jan. 6 commissionMORE (Calif.), wrote in a letter to Pelosi.

Rep. Brian MastBrian Jeffrey MastThree GOP lawmakers fined 0 for flouting House floor mask rulesRepublicans defy mask rules on House floorTapper battles GOP lawmakers over criticism of Afghan vet’s Electoral College voteMORE (R-Fla.) was spotted sitting in the House chamber without a mask. And Rep. Marjorie Taylor GreeneMarjorie Taylor GreeneThe Memo: What now for anti-Trump Republicans?Rep. Marjorie Taylor Greene says she’s meeting with Trump ‘soon’ in FloridaQAnon site shutters after reports identifying developerMORE (R-Ga.) tweeted a photo of herself and three other lawmakers maskless on the House floor, arguing that masks were oppressive.

House Democratic leaders defended their decision to continue requiring masks on the chamber floor, saying they are simply following the guidelines put forward by the Capitol physician. Some jabbed back at GOP critics, arguing that if Republicans want a return to normalcy, they should encourage their reluctant colleagues to get inoculated.

CDC guidelines are … very clear that workplaces can decide their own unique characteristics based on their conditions. And I think we would agree that the House floor is unique condition, especially with Republicans admitting that only 75 percent of their members are vaccinated, said Rep. Pete AguilarPeter (Pete) Ray AguilarCapitol Police watchdog calls for boosting countersurveillanceTSA chief cites ‘substantial increase’ in firearms at airportsDemocrats seek to keep spotlight on Capitol siegeMORE (Calif.), vice chair of the House Democratic Caucus.

House Majority Leader Steny HoyerSteny Hamilton HoyerBacking Biden, Pelosi calls for cease-fire in IsraelWhat’s a party caucus chair worth?House fails to pass drug bill amid Jan. 6 tensionsMORE (D-Md.) offered a similar assessment.

If you look at the Office of the Attending Physicians guidance, one of the cogent parts of that was that not every member has been [vaccinated], he said.

Im glad that were back. Were getting back to normal. The presidents pleased about that, Hoyer added. But what we want to make sure we dont have is another, in effect, relapse, a surge.

The dynamic is different just across the Capitol.

The attending physician sent a letter to Senate leadership, which was shared with Senate offices last week, saying that if individuals were fully vaccinated you can resume activities that you did prior to the pandemic.

You can resume activities without wearing a mask or staying 6 feet apart, the physician wrote in the letter, a copy of which was obtained by The Hill.

That guidance, unlike a letter sent to House lawmakers, does not carve out an exemption for the Senate floor, where masks were encouraged but never required. Senators have largely stopped wearing masks, though a handful of GOP senators and a more significant portion of Democrats have been spotted sporting them around the Capitol and during votes.

Senators tipped their hand at the move toward normalcy, even as the Capitol remains closed to tours and the office buildings are still reserved for staff or planned-in-advance meetings.

Its nice to see some smiling faces again, Sen. John ThuneJohn Randolph ThuneSenators shed masks after CDC lifts mandateLawmakers bicker over how to go after tax cheatsGOP split on counteroffer to Biden’s spendingMORE (S.D.), the No. 2 Senate Republican, told reporters who were largely not wearing masks at a weekly leadership press conference, adding that it was proof and evidence that vaccines work.

Whether to wear a mask for senators is largely ad hoc.

If Im going into a meeting thats going to be a lot of us in one smaller area, I wear a mask, said Sen. Mike RoundsMike RoundsSenators shed masks after CDC lifts mandateGOP split on counteroffer to Biden’s spendingDemocrats, GOP agree on one thing: They’re skeptical of a deal MORE (R-S.D.), opening up his jacket pocket to show that he was carrying a mask with him. If Im going down the halls like this, Im not wearing a mask.

Senate Majority Leader Charles SchumerChuck SchumerGOP splits open over Jan. 6 commission voteSenators shed masks after CDC lifts mandatePro-tax millionaires protesting in front of Bezos’s homesMORE (D-N.Y.), after being spotted wearing a mask immediately following the new CDC guidance, has been maskless this week.

Republicans have taken notice.

Sen. Chuck GrassleyChuck GrassleyConservatives launch ‘anti-cancel culture’ advocacy organizationLawmakers bicker over how to go after tax cheatsOn The Money: Biden says workers can’t turn down job and get benefits | Treasury launches state and local aid | Businesses jump into vax pushMORE (R-Iowa), asked about masks, said: I think I took my cue from when Schumer came to the floor … without a mask.

Mike Lillis contributed.