Overall, 59 percent of Romney voters in the Republican primaries lived in the states carried by President Obama. Those states hold 50 percent of the delegates to the Republican National Convention, even though they contain just 19 percent of Republican senators. Just 11 percent of House Republicans hail from districts that voted for President Obama.

For all the legitimate attention that will be given to questions about whether an establishment favorite like Mr. Bush can win over deeply conservative voters, there are just as many questions about which conservative candidate can win over blue-state Republicans. Mr. McCain and Mr. Romney won every blue-state primary in 2008 and 2012, making it all but impossible for their more conservative challengers to win the nomination.

“There’s no question the presidential trail goes through places that congressional Republicans don’t always have to go,” said Ari Fleischer, the first White House press secretary for George W. Bush, the last Republican to win the party’s nomination largely because of strength in red-state primaries. Mr. Bush struggled in blue states, losing early primaries in New Hampshire and Michigan, but still secured the nomination.