Republicans are famously divided on immigration reform, but Democrats pretty much unanimously support it. There's a reason for that.

In stark, partisan political terms, continuing the high level of immigration of recent decades, and certainly increasing immigration as envisioned by many reformers, will result in more Democrats winning more elections in coming years.

"Immigrants, particularly Hispanics and Asians, have policy preferences .. .more closely aligned with progressives than with conservatives."

"The enormous flow of legal immigrants into the country — 29.5 million from 1980 to 2012 — has remade and continues to remake the nation's electorate in favor of the Democratic Party," concludes a new report from the Center for Immigration Studies, which opposes comprehensive reform proposals like the Senate "Gang of Eight" bill. "As the immigrant population has grown, Republican electoral prospects have dimmed, even after controlling for alternative explanations of GOP performance."

In the report, author James Gimpel, a University of Maryland professor, looks at the immigrants who have come to the United States in recent decades and those likely to come in the future. Through a lot of complicated statistical analysis and close reading of previous studies, he comes to the same conclusion as anyone who has looked through exit polls in the last 30 years: Immigrants tend to vote Democratic.

A 2012 study of 2,900 foreign-born, naturalized immigrants cited in the report showed that about 62 percent identified themselves as Democrats, while 25 percent identified as Republicans, and 13 percent identified as independents. At this moment, according to the report, there are an estimated 8.7 million immigrants in the U.S. who are eligible for naturalization. Not all will become voting citizens, but somewhere between 50 percent and 60 percent will. And it's a sure bet that a majority will identify themselves as Democrats.

Gimpel cites several reasons why future immigration will likely mean more Democrats. The first is that "immigrants, particularly Hispanics and Asians, have policy preferences when it comes to the size and scope of government that are more closely aligned with progressives than with conservatives." Those preferences have expressed themselves in a two-to-one party identification advantage for Democrats in those groups.

Another reason is that the arrival of immigrants, whose ranks include substantial numbers of the poor and unskilled, increases income inequality in the areas they choose to live. "It is from areas of higher income inequality," writes Gimpel, "that we find the most support for a robust government with an expansive regulatory and redistributive role in the economy, among all citizens, not just immigrants." That will likely mean more electoral success for Democrats.

Gimpel found that the partisan impact of immigration "is relatively uniform throughout the country — from California to Texas to Florida." If immigrants arrive in large numbers, areas that are already Democratic become more so, while areas that are Republican become more Democratic. That applies to Texas and other red-state strongholds as much as anywhere else.

The political changes immigration has brought to some of the nation's largest counties are striking. Broward County, Fla., was made up of 11.1 percent immigrants in 1980 and is 31.2 percent immigrant today. It was 55.9 percent Republican in 1980 and 32.4 percent today. San Bernardino County, Calif., was 7.7 percent immigrant in 1980, and 21.4 percent today. It was 59.7 percent Republican back then, and is 46.2 percent today. Clark County, Nev., was 7.6 percent immigrant in 1980 and is 21.9 percent today. It was 59.8 percent Republican then and is 42.6 percent today.

To some Republicans, Gimpel's findings will be just more proof that the GOP must support comprehensive immigration reform as it seeks to build ties with immigrant communities. But Gimpel suggests that doesn't matter. "The decline [in Republican identification] does not seem to vary with the local Republican Party's position on illegal immigration," he writes.

Of course, Republicans are also seeking other ways to appeal to Hispanic voters. Gimpel suggests that if they succeed, it won't be anytime soon. "Entrenched patterns of party loyalty change very slowly, over decades," he writes, "and are not ordinarily subject to wild swings in response to campaign stimuli."

The bottom line is that more immigration favors Democrats; there is no prediction of Democratic electoral ascendancy that doesn't rely on demographic factors as the main engine of the party's dominance.

Even if no changes are made to increase immigration, Republicans face a daunting, long-term task of trying to win the loyalty of immigrant voters. With about 30 million who have arrived here legally in the last three decades, plus about 12 million who are here illegally now but could well become voters someday, plus their natural-born citizen children on the way in the future — it's a hugely important assignment for the GOP.

But it's also reasonable for Republicans not to support policies that could worsen their electoral prospects, if not doom their party to decades of defeat. After all, Democrats are certainly acting in what they perceive to be their party's best political interests.

There are plenty of policy reasons to be on one side or the other in the immigration debate. But in the end, it's all politics. This new report suggests Republicans should understand that as well as Democrats.