WASHINGTON — Self-professed conservatives have long outnumbered liberals in America, but the gap has narrowed significantly in the last four years, particularly on social issues, a shift that could harm GOP prospects in future elections.

On social issues, the number of people who identify themselves as liberal is now almost equal to the share who say they are conservative, according to the latest polling by Gallup. For years, conservatives held an advantage.

About one-third of Americans identify with either group. Another third call themselves moderates on social issues. As recently as 2010, conservatives had a 17-point advantage over liberals on social issues in Gallup’s polling.

A similar shift has taken place on economic issues, although the conservative advantage remains bigger in that realm.

Just as a rising conservative tide helped Republicans in 2010, a waning one – if it continues – could pose problems for the party in future elections.

Measuring how people identify themselves ideologically doesn’t necessarily reveal how they will vote on specific issues or candidates. Many Americans do not have consistent ideologies. But the shift in how Americans identify themselves on social issues has coincided with stronger support for liberal positions on issues including same-sex marriage, the death penalty and legalized marijuana.

Shifts in ideological identification do provide some clues to voting patterns. Typically, the public becomes more liberal when conservatives hold power and more conservative under liberal administrations –seemingly reacting against the perceived excesses of whichever party holds power.