New polling data sees the Ohio Senate race continue to tighten while Pennsylvania Sen. Bob Casey, Jr. (D) expands his lead over what appeared to be a recent changing trend.

Earlier this week, the Morning Consult research organization released their new data (8/30-9/8; 1,558 OH likely voters; online panel) and also finds the Ohio Senate race getting closer. The MC ballot test, 46-43% in Sen. Sherrod Brown’s (D) favor, is consistent with other recently released results.

The previous Emerson College survey (9/3-5; 945 OH likely voters; multiple sampling techniques) published late last week produced a 46-44% split between Sen. Brown and Republican nominee Bernie Moreno. Previously, ten consecutive polls found Brown’s edge at or around five percentage points, but he only secured majority support in two of the studies.

Despite former President Donald Trump consistently performing well in the Buckeye State, typically running nine to ten points ahead of either President Joe Biden or Vice President Kamala Harris, Sen. Brown had shown resiliency in continuing to lead Mr. Moreno by a consistent five percentage points. But, even at a +5 polling result, the margin may not be enough.

Since research shows that Sen. Brown over-polled in his previous race (2018) – his polling lead was 13 points headed into the election according to the Real Clear Politics polling Archives, but his victory margin was six – the current campaign is likely to get even tighter as we continue to move toward election month.

If a similar over-polling pattern is present in this race, then Sen. Brown could well be in trouble and Republicans may be positioned to score an upset. In-person early voting begins in Ohio on October 8th.

At the end of August, four Pennsylvania Senate race polls found the contest between Sen. Casey and Republican businessman David McCormick falling into the toss-up realm with a ballot test range of 0-4 percentage points.

Now, we see a different story. The latest polls find Sen. Casey rebounding in an even newer series of studies. During the period of September 3-9, four polls were released (CBS News; co/efficient; Morning Consult; and Redfield and Wilton Strategies) that produced data revealing Casey leads of 7, 9, 9, and 8 percentage points, yet the Senator never reached 50% in any.

Based upon 2018 polling, the last time that Sen. Casey was on the ballot, we see an over-poll effect for him but not as dramatic as the Ohio pattern. Based upon eleven polls conducted from six different pollsters, Sen. Casey recorded a mean average advantage of 15.6 percentage points over then-US Representative Lou Barletta (R) and defeated him by 12.9 points.

Therefore, in the 2018 race, Sen. Casey over-polled by just under three full points. If that pattern again exists in 2024, a three point polling miss in Mr. McCormick’s favor could prove important.

It is arguable that the polling error factor could be even greater this year because the voter registration imbalance is quite different when compared to 2018. The last time Sen. Casey was on the ballot, Democrats had an 821,416 person registration advantage over Republicans. The latest report from the Pennsylvania Secretary of State shows that the Democratic advantage has dwindled to just 168,850 individuals, and Republicans outnumber Democrats in 54 of the Commonwealth’s 67 counties.

Coupled with even a slight over-poll for Sen. Casey, the difference in registration could be accounting for what should be a much closer finish in 2024. Irrespective of the polling swing and the partisan registration counts, Sen. Casey must still be favored to win a fourth term in November, though his victory margin could be substantially smaller.

Early voting in Pennsylvania begins next week in some counties, but the localities have leeway as to when to commence their in-person voting.