Any conversation about the nation’s immigration system is likely to include a version of this basic question: Why don’t illegal immigrants just get in line and come here legally?

One big reason: many don’t qualify to get in any such line. And for those who do, the line can be extraordinarily long.

About 4.3 million immigrants who have relatives in the United States were waiting for family-sponsored visas to live here legally as of Nov. 1, according to a U.S. State Department report. Some have been waiting for years, while others have waited decades.

Those who believe immigrants are an asset to the country are calling on the government to lift caps on the numbers of visas available each year. President Barack Obama has proposed doing just that, at least until the current backlog is cleared.

Those who believe admitting more immigrants will weaken the nation’s economy and social fabric oppose the idea.

Many experts say that, whatever one’s philosophical stance, such long waits encourage illegal immigration. Even if Congress addresses other aspects of the immigration system, unless it adjusts the visa limits, the same problem that exists today — millions of people living illegally within the nation’s borders — is likely to recur in the future, they say.

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