Let’s unpack what he meant:

Mr. Rubio helped make possible the Senate’s proposed $2,000 child tax credit, which had been $1,650 per child in the version of the bill unveiled by the Senate Finance Committee. Mr. Rubio had wanted to make that tax credit even more generous, allowing it to be refundable against payroll taxes for parents who don’t earn enough to qualify for the entire $2,000. In the current Senate bill, just $1,100 is refundable.

Mr. Rubio lost that bid in a Senate fight last week. His amendment, offered with Sen. Mike Lee (R., Utah), would have paid for an expanded tax credit by setting the corporate tax rate at 20.94%, instead of the 20% rate proposed in both Senate and House bills. It was defeated in a 29-71 vote.

Now, Republican negotiators are meeting behind closed doors to iron out differences between House and Senate bills. They must decide whether to back the House approach to the child tax credit, set at $1,600, or the Senate’s $2,000 child credit, which phases out at incomes of up to $500,000.

Mr. Rubio’s message is two-fold: He wants the Senate’s version of the child tax credit to prevail. But he’s also worried that negotiators will opt for a corporate tax rate of higher than 20% to free up room for other priorities. Mr. Rubio is saying that if the corporate rate goes any higher than 20%, his priority—a more generous child tax credit—can’t get lost in the mix of competing demands.

The risk is that Mr. Rubio might vote against the tax bill, which passed 51-49 last week and can only afford one more Republican defection if the final version is to become law. (Vice President Mike Pence can break a 50-50 tie.) It isn’t clear whether Mr. Rubio would withhold his vote. He could have voted against the Senate tax bill last week to demonstrate his leverage, but he didn’t.

A spokeswoman for Mr. Rubio didn’t respond to a request for comment.