The GOP’s chances of flipping one of Maryland’s U.S. Senate seats are evaporating as support for Governor Hogan slips in the deeply Democratic state, following in the path of many popular “crossover” governors — those from a party a state does not normally support who have tried to move into the Senate.

A Public Policy Polling survey released Monday found that the Democratic Senate nominee, the Prince George’s county executive, Angela Alsobrooks, was leading Mr. Hogan by 45 percent to 34 percent in the state, with 5 percent saying they supported someone else.

When asked to choose just between Ms. Alsobrooks and Mr. Hogan, this gap shrinks to eight points, 48 percent for Ms. Alsobrooks and 40 percent for Mr. Hogan.

While an eight-point lead in a face-to-face matchup might sound relatively modest in Maryland, a state President Biden carried by 33 points in 2020, it represents a significant shift from polling conducted early in Mr. Hogan’s candidacy.

A survey conducted in the weeks after Mr. Hogan’s March announcement that he would seek election to the Senate found that he was leading any Democratic candidate in the race. The Washington Post and University of Maryland survey found that Mr. Hogan led Ms. Alsobrooks 50 percent to 36 percent, suggesting that the electorate has swung toward Democrats in recent weeks.

That swing will be stinging to Republican efforts to flip the seat, which have relied on Mr. Hogan being a singular character in Maryland politics — a popular Republican who has proven his ability to win statewide.

Senator McConnell was quoted saying as much by Bloomberg’s Erik Wasson, who reports that Mr. McConnell personally worked to recruit Mr. Hogan, calling him “the best” recruit of the cycle because it made the GOP competitive in “a state nobody thought we could win.”

“He’s a proven winner,” Mr. McConnell said.

President Trump also rallied behind Mr. Hogan, despite Mr. Hogan’s historically contentious relationship with the former president, saying that he’d like to see Mr. Hogan win, even after the former governor called on Americans to respect the verdict in Trump’s New York fraud trial.

The drop in support for Mr. Hogan despite Republican efforts to unify behind him mirrors the fate of many candidates in Mr. Hogan’s mold.

Take the example of the Democratic governor of Tennessee between 2003 and 2011, Phil Bredesen. Mr. Bredesen won the governorship in 2002 with about 51 percent support and won re-election in 2006 with some 69 percent of the vote. Yet when Mr. Bedesen ran for Senate in 2018 he lost by just more than 10 points to Senator Blackburn.

A popular former Democratic governor of Montana, Steve Bullock, provides another example of crossover governors falling flat in a Senate race. In his 2012 election to the governorship, Mr. Bullock won over the Republican candidate, Congressman Rick Hill, by about two points. 

In 2016, Mr. Bullock won re-election with just more than 50 percent support. Yet in 2020, Senator Daines defeated Mr. Bullock by 10 points, 55 percent to 45 percent.

Republican crossover governors have fared similarly poorly in attempting to parlay their popular governorships into Senate bids.

The first Republican elected governor of Hawaii since 1959, Linda Lingle, who was also the first female governor of the state, won by four points in the 2002 gubernatorial election. In 2006, Ms. Lingle won re-election by the largest margin a governor has ever won in the state, 28 points, defeating the Democratic nominee 63 percent to 35 percent. When she attempted to run for the Senate in 2012, though, Ms. Lingle lost to Senator Hirono by 26 points, 63 percent to 37 percent.

While Mr. Hogan might be able to revive his chances of making Maryland competitive, it’s clear he is fighting against a trend. As of 2024, there are eight states where the governor is from a different party than the one the state supported in the 2020, meaning 16 percent of governors are crossover governors.

In the Senate, though, there are just five crossover members, one of whom, Senator Johnson, is from the perennially competitive state of Wisconsin. Another, Senator Manchin, is retiring.

In other words, crossover senators are rare and becoming rarer and, while crossover governorships are becoming less common as well, partisanship appears to play a bigger role in Senate races.

A 2023 analysis of the phenomenon from the University of Virginia Center for Politics’s Sabato’s Crystal Ball explained the phenomenon in terms of senators’ lack of ability to separate themselves from national issues.

“Governors likely have a little more ability to separate themselves from national political factors, and sometimes they are out of step from their national parties — but in-step with state electorates — on important issues,” editors Kyle Kondik and Miles Coleman wrote.