FL-21 · House

The district, in brief

Florida's 21st is on a map first used in 2026; the text says nothing about where the new lines run. The incumbent is Republican Brian Mast, re-elected with 61.8% of the vote in 2024.

Mast is the Republican nominee and the text lists no primary opponent. The Democratic nominee is James Martin, a lieutenant commander in the U.S. Coast Guard Reserve, who won his primary 50.4% to 49.6%, 24,054 votes to 23,692, over firefighter Bernard Taylor, who had out-raised him $993,380 to $579,319. Pia Dandiya withdrew to run in the 22nd district. An independent, Alexander Cooke, a former mayor of Juno Beach, has declared, David Fabrikant is a write-in, and an MGTOW party candidate, Amr Metwally, failed to qualify. As of June 30, 2026 Mast had raised $4,308,890 with $2,928,458 on hand; as of July 29, 2026 Martin had raised $579,319, of which $150,001 was self-funded, with $188,041 on hand.

Every rater listed calls the seat R: Decision Desk HQ (July 14, 2026), the Cook Political Report (February 6, 2025), Inside Elections (May 5, 2026), Sabato's Crystal Ball (April 10, 2025) and The Economist (May 6, 2026). The text records no general-election poll.

A paragraph quoting a model forecast from the day this brief was written is omitted; the current forecast is at the top of this page.

The facts, each beside its sentence

Not in the source: The text describes nothing about the new district lines, and gives no presidential results on any lines.; No Republican primary result is given; Mast is listed as nominee with no opponents.; No general-election poll is in the text, and the model's basis says no district polls.; No general-election ratings finer than 'R' are given (no Lean/Likely/Safe).

Read from the district's section of Wikipedia's state page (CC BY-SA), archived as 27e2e54e6981; written by claude-fable-5-1 (batch reader) on 2026-09-30 and checked mechanically: every quote occurs verbatim in the archived text, every number in the brief occurs in the inputs. No probability here is the reader's.

Who holds the seat now

Brian J. Mast (Republican), in this chamber since 2017, 5 terms. Committees: House Committee on Transportation and Infrastructure; House Committee on Foreign Affairs (Chair).

From the congress-legislators dataset (CC0), archived as ae8cb9477c48. For a Senate race this lists both of the state's senators; the one whose term ends in the election year holds the seat on the ballot. For a House district redrawn for 2026 it is the member elected under the old lines with this number, who may be running elsewhere; the dossier above names who is running here.

How it has voted for president

A shift of 8.9 points toward the Republicans from 2016 to 2024. county returns placed on the current lines by 2020 block population; MEDSL county file (2016), an MIT-licensed county file (2020, checked against certified state results) and MEDSL precinct returns (2024). For a district split across counties this is an estimate, and where the state published the exact 2024 figure for these lines, that exact figure is the one the model uses.

Who are the voters?

Of about 579,742 adults 25 and over. In 2020 the district voted 45.5% Democratic (two-party) for president. With the same preferences and a 2022-style midterm turnout, it would vote 45.8% Democratic: the midterm electorate alone moves the margin 0.5 points toward Democrats. Group shares are estimated from how results vary across the state's census tracts (ecological inference, anchored to Pew's 2020 validated-voter figures) and calibrated to this district's actual 2020 vote; turnout is the Census survey's state rate by race and national rate by education, whose state samples carry margins of several points.

Not answered here: how each group will vote in 2026. These are 2020 preferences; this profile is description, not yet an input to the forecast.

The money

FEC bulk candidate summaries through July 29, 2026; $99,500 of independent expenditures for or against these candidates this cycle (FEC Schedule E, types 24E and 24A), which is the parties' and outside groups' own revealed view of whether this seat is in play.

The district in numbers

ACS 5-year 2023, assembled from census tracts by the 2020 population of their blocks inside the current lines, because the Bureau has not tabulated these lines; medians are population-weighted means of tract medians. Unemployment is the BLS county series placed on the district by block population, not seasonally adjusted, so changes are same-month comparisons. Income, transfers and jobs are BEA county accounts placed the same way, compared with the average district. Every other figure is a named column of a named table in the Bureau's summary file, archived before it was read. This is what a reader knows about a district before looking at a poll; the model does not yet use it, and whether it should is a registered question.

Who is favoured, and by how much?

The 90% interval runs from -25.0 to +4.9 points, D-R. It is wide because this district has never been polled.

What is this built on?

A model trained on every contested House race 2012-2024 predicts this district's lean from its own record: presidential and House results on the current lines, incumbency, both candidates' prior offices, outside and party money before 1 October, demographics and public-assistance shares from the Census release before the election, local unemployment and income, and religious tradition. The national margin comes from presidential approval, the economy, the midterm penalty and the generic ballot corrected for its measured lean. Draws share national and state errors, so the seat total and this district come from the same simulation.

Not answered here: district polls, which are not yet folded in.

Why does the model say this?

Each figure is how far this district's predicted lean moves when that one feature is set to the typical value across 2026 districts, everything else held: 2024 House lean: 1.7 pts toward the Republican; experience gap (-4 levels, D minus R): 1.6 pts toward the Republican; wages share income (27.3%): 0.8 pts toward the Republican; PAC money balance: 0.7 pts toward the Republican; 2024 presidential lean (R+9.5): 0.7 pts toward the Republican.

How sure is it, and of what?

Total sd 9.10 points, made of the seat's own volatility 90%, correlated polling error 10%. The decomposition is stored with the forecast rather than reconstructed, so a probability whose dominant input is a placeholder is visibly that.

Not answered here: a nowcast. 'If ballots were cast today' needs something that observes today, and for a House district nothing does. Publishing the election-day figure under both labels would make one forecast look like two.

How was this number made?

Run 0b33151b-a949-47f1-b845-564833a2b74d, house_district_prior/0.2.0, as of 2026-10-02. Promoted by a pre-registered gate: on 2016-2022 its Brier score was 0.036 against 0.049 for the previous method, and on a single 2024 run 0.023 against 0.032 (experiments/district_gate_result.md, with an independent review).