“This, America, is absolutely irresponsible. You cannot be near other people while waiting for coronavirus test results,” said Sen. Kirsten Sinema (D-AZ).
“No one is too important to disregard guidance to self-quarantine pending test results,” added Sen. Martha McSally (R-AZ).
Paul's office staff - which was manning his Twitter feed - denied that the Kentucky Republican had done anything wrong by being in public while waiting for his test results.
But it clearly raised questions.
Two GOP Senators soon went into self-quarantine - Sen. Mitt Romney and Sen. Mike Lee, both from Utah - as both had been in close proximity to Paul in recent days.
With Paul on the sidelines - along with Sen. Cory Gardner (R-CO) and Sen. Rick Scott (R-FL) - five different GOP Senators are now unable to participate in Senate votes and debate.
At the White House, the President lamented the diagnosis for Paul as he said nice words about the Kentucky Republican.
For Romney, it was not the same reaction.
“Romney's is isolation?” the President said to a reporter.
“Gee, that's too bad,” Mr. Trump said in a mocking tone.
Paul joins two House members - Republican Mario Diaz-Balart of Florida, and Democrat Ben McAdams of Utah - in testing positive for the virus, which has spread around the world in recent months.
None of the three were thought to be in any immediate health danger.
Federal health officials have largely said that people without any symptoms should not be tested, but there is concern among some health professionals that otherwise healthy people could also be carrying the virus - and spreading it.
Last week, Paul was at work on Capitol Hill.
Paul had generated headlines earlier this month when he slowed work on an $8 billion Coronavirus response bill approved by the House.
The Kentucky Republican tried but failed to force the Congress to pay for the plan wit cuts in other areas of the budget.
Paul was the only Senator to vote against the first Coronavirus bill, and one of eight to vote against a second plan approved last week.
With Paul and his four other GOP colleagues on the sidelines, the GOP majority in the Senate is down to 48-47, instead of the usual 53-47 split.