Polls point to a possible Democratic takeover of at least one chamber in November. But the historical market penalty is modest and applies most directly to a full sweep. Here's what investors should, and shouldn't, take from the data.
U.S. Bancorp's research puts the difference at 0.99%. When a midterm produced a Republican president and full Democratic control of Congress, the S&P 500's three-month return averaged 0.99% below its return across all historical periods.
That is not a forecast of a market decline in 2026. It's a past average tied to one political outcome, and current polling does not guarantee that outcome. The research points to economic growth and inflation as more lasting influences on returns.
Carson Group's historical data for midterm years since 1950 show that October was the S&P 500's strongest month on average, with a 3.0% gain and positive returns in 73.7% of cases. November averaged a 2.7% gain and finished higher in 78.9% of those years.
What the forecast says
On September 20, the FiftyPlusOne 2026 Congressional Forecast gave Democrats a 97% chance of winning the House and a 64% chance of taking the Senate. An update on September 24 raised those odds to 98% and 69%. Those are probabilities, not results. If Democrats win one or both chambers while President Trump remains in the White House, the result would be divided government in Washington.
The distinction matters. U.S. Bancorp's 0.99% comparison covers a Republican president with Democrats in control of both chambers. It does not cover every Democratic gain in the House or Senate. The finding is narrower than a general claim about election outcomes.
Oppenheimer's September 22, 2026 market review noted that midterm years have historically brought greater volatility in the middle and later parts of autumn, a pattern it tied to seasonal turbulence rather than the election result itself. Since 1948, midterm years averaged 4.6% annual S&P 500 growth, compared with 11.2% in other years of the presidential cycle.
A short window, not a market rule
Even in the full-sweep scenario, the historical difference covers just three months. It does not show that a Democratic Congress causes stocks to fall, or that the pattern will repeat in 2026. Markets respond to the broader economy as well as election results. U.S. Bancorp's 2024 research found that growth trends and inflation generally matter more to S&P 500 returns than which party wins.
The same analysis found that the S&P 500 has performed better in the year after midterm elections than in non-midterm years, no matter which party controls Congress. Relief after election uncertainty passes could help explain the pattern. It does not guarantee a post-election rally. These are historical averages, not a reliable trading calendar.
Investors comparing political headlines with company fundamentals can look to this quality-stock analysis. It focuses on business strength rather than election timing. A short political window and a long investment horizon call for different ways of thinking about risk.
Keep the time horizon in view
The longer record has less to do with who holds office. Since 1928, the S&P 500 has delivered annualized returns of about 10% through presidencies and Congresses of both parties, under unified and divided government. Vanguard S&P 500 ETF (VOO) has returned 15% annualized since September 2010. That period included four midterm elections and multiple presidencies. Neither figure guarantees future performance.
A Democratic Congress could try to undo policies tied to "Trumpflation." The research commentary links those policies to rising Treasury yields. That is one possible force on stock prices, not a certain market outcome. Other economic developments could push prices either way. Political views may shape how investors read these possibilities, but they cannot settle how markets will respond.
Annualized returns express a compound rate over time. They do not mean the index gained that amount every year, or that investors avoided losses along the way. Short-term returns can differ sharply from long-run averages. The three-month election comparison is an especially limited guide to an individual portfolio.
The signal is modest and conditional. Elections can matter, but this historical pattern alone is too narrow to justify a major change to a long-term plan.
