Sen. Kyrsten Sinema (D-Ariz.) is negotiating the size and scope of President Biden’s $3.5 trillion budget plan armed with her own spreadsheets about the costs and tax hikes needed for each program, people familiar with the matter tell Axios.

Why it matters: While Sen. Joe Manchin (D-W.Va.) is getting attention for balking at a $3.5 trillion top-line price tag, Sinema’s accountant-like focus on the bottom line will be equally important to winning the votes of them and other key Democrats.

Sinema’s intense interest in the numbers also suggests she’ll be a formidable foil for progressives — like Sen. Bernie Sanders (I-Vt.) — who are working to make the spending bill as big as possible.