The situation is nearly as severe in the House. Because Hispanics generally live in the same areas, Congressional map makers can easily draw heavily Hispanic and heavily Democratic districts. (Sometimes, the Voting Rights Act pushes the map makers to create such districts; sometimes neutral map makers attempt to preserve so-called communities of interest; and sometimes Republican officials deliberately draw such districts to minimize the number of Democratic seats.)

As a result, half of all Hispanics live in just 65 of the nation’s 435 congressional districts. In districts held by House Republicans, Hispanics represent only 6.7 percent of eligible voters. There are a handful of competitive House districts with a large number of Hispanic voters, making the Hispanic population share in the House battlegrounds, at 7.4 percent, slightly higher than the Senate share. But with a healthy edge in the House, the G.O.P. can afford to lose the handful of competitive seats where Hispanics represent a meaningful share of eligible voters.

Hispanics are earning more clout in presidential elections. It is in those elections, not in the fight for Congress, where Hispanics could ultimately force the hand of Republicans. Hispanic voters have already helped move Nevada and New Mexico into the lean-Democratic column, costing Republicans 11 electoral votes won by George W. Bush in 2004.

Hispanics could make Republican presidential victories even more difficult depending on what happens in Florida, where the Hispanic population has grown rapidly and become more Democratic. If the Republican Party can’t make gains among Florida’s Hispanic population, it will face a very challenging pathway to the presidency.

Yet Hispanics are still punching beneath their weight in presidential elections. ln battleground states, Hispanics represent more than 5 percent of eligible voters in only Florida, Nevada and Colorado. As a result, President Obama would have easily won re-election even if he had not won the Hispanic vote; he would have won the Electoral College even if he had lost all three of those states.

In time, the political underrepresentation of Hispanics will end. The Hispanic share of the electorate will steadily increase. Most of today’s Hispanic children were born in the United States; 94 percent are citizens. As they reach voting age, the Hispanic share of eligible voters will begin to catch up with the Hispanic share of the population, although it may take a generation or longer. The Hispanics who are already eligible to vote will become more likely to participate with age.

But for now, Hispanic voters will struggle to get their voices heard. Their influence in presidential elections is rising, but the Republican senators and House members who are needed to advance immigration reform are insulated from the fraction of American Hispanics who are eligible to vote and participate in elections.