Even against Jeb Bush and Marco Rubio, Hillary Clinton begins with a landslide-level advantage among Hispanics.

This early in the game, that's hardly determinative. But it means Republicans begin the 2016 presidential race with a hill to climb against the former secretary of state and presumptive Democratic nominee. Bush and Rubio, two Florida Republicans presumed to be their party's strongest contenders for the Hispanic vote since they have Hispanic ties and are on record as supportive of immigration reform in some form, trail Clinton 66 percent-28 percent and 63 percent-32 percent, respectively, in the race for the key bloc, according to an NBC News/Wall Street Journal poll.

"At the national level, as time progresses and as that population segment grows, they're just going to have increasing importance. That's a pretty straightforward trend," GOP pollster David Winston said.

In other words, it would become harder for Republicans to win the White House if the dominance of the GOP-friendly white vote continues to recede and the party's share of the Hispanic vote remains stuck around 30 percent. That's partly because the dynamic diminishes the number of states the GOP can compete for, leaving the party no room for error as it seeks to assemble 270 Electoral College votes.

In 2012, President Obama won 73 percent of the Hispanic vote to Republican nominee Mitt Romney's 27 percent. Doing better with Hispanics, who are a diverse group and hardly homogenous — ethnically or otherwise — might have given Romney a better shot in the battlegrounds of Colorado, Florida and Nevada, where Hispanics measured 14 percent, 17 percent and 18 percent of voters, respectively.

Flipping those states' combined 44 electoral votes wouldn't have won Romney the election, but it would have put him within striking distance. Yes, even winning Ohio and Virginia, where Hispanics accounted for only 3 percent and 5 percent of voters in 2012, respectively, wouldn't have mattered unless he also won Florida and either Colorado or Nevada.

That is a dilemma the Republicans could face again in 2016, and why improving their performance with Hispanics is crucial. Indeed, when the GOP performs well with Hispanics, they tend to win. In 2004, President George W. Bush won 44 percent of the Hispanic vote on his way to being re-elected. In 2010 and 2014, both wave midterm elections for the Republicans, they won 38 percent and 36 percent, respectively.

Is there a magic number? Not necessarily. But since 1992, Republicans won every national election — midterm or presidential — in which they picked up at least 35 percent of the Hispanic vote, except one. When losing, Republicans secured anywhere from 26 percent to 30 percent. Only in the 1998 midterms, when Democrats secured narrow congressional gains in the aftermath of President Bill Clinton's impeachment, did Republicans earn an acceptable 35 percent of the Hispanic vote but still came out on the losing end.

Michelle Diggles, a political analyst at the centrist Democratic think tank Third Way, manipulated the data to figure out what would have happened if the Republicans had done as well with ethnic minorities in 2012 as Bush did in 2004 — and how Bush would have fared in 2004 had the demographic makeup of the electorate been similar to three years ago but his vote share constant.

"If we look just at the national popular vote, President Bush would have won re-election in 2004 even if the electorate mirrored the racial and ethnic distribution of 2012," she said. "In fact, if President Bush would have won racial and ethnic groups by the same percentage as he did in 2004, Bush would have won the 2012 election 49.68 percent to 49.02 percent."

Republicans focused on the Hispanic vote in 2016 have pointed to Jeb Bush, 62, and Rubio, 43, as two candidates who could make inroads with the group.

Bush, the former two-term governor of Florida, speaks fluent Spanish and is married to a Mexican American. Rubio, Florida's junior first-term senator, speaks fluent Spanish and is a first-generation American born to parents who were born and raised in Cuba. Both candidates favor broad changes to immigration law and are open, under certain conditions, to allowing illegal immigrants to remain in the U.S.

But the NBC News/Wall Street Journal poll shows they have almost as much work to do with Hispanics as their competitors for the GOP nomination. Sen. Rand Paul of Kentucky trailed Clinton among Hispanics 68 percent to 28 percent — mirroring Bush. Wisconsin Gov. Scott Walker lagged Clinton 69 percent to 24 percent. Only Rubio cracked 30 percent and pulled Clinton below 65 percent. The survey of 1,000 registered voters, conducted April 26-30, had an error margin of 3.1 percentage points.

Walker, a frontrunner for the nomination, used to support immigration reform policies that include a pathway to legal status for illegal immigrants. But the governor has changed his position since becoming a 2016 contender. Walker now opposes "amnesty" in any form and has even expressed support for tying legal immigration to its impact on the economy and American workers' wages, a position some view as being equal to placing more stringent limits on legal immigration.

Clinton's campaign promise to pursue what is essentially blanket amnesty for America's 11-12 million illegal immigrants, made last week during a campaign appearance in Las Vegas, could complicate Republican efforts to attract more Hispanic votes. Clinton's amnesty push, including vows to pursue more generous legalization policies through executive action if necessary, could put Republicans on the defensive, or, at the very least, give greater voice to immigration hardliners.

Supporting a path to citizenship isn't a prerequisite for Republicans to improve their relationship with Hispanic voters. But they have to avoid taking hostile positions, as Romney did in 2012 when he took a hard line on immigration and suggested that illegal immigrants should "self deport" from the U.S.

"Immigration is important as a symbolic issue," said Republican pollster Whit Ayres, who has studied this issue. "Not only the tone, but the substance, when discussing it, is critical."