The Democrats are outpacing Republicans in voter registration.

As Election Day draws near, the number of Democratic voters is growing in key battlegrounds, compared to voters affiliating with the Republican Party.

It's a crucial development that could tip the scales in the race for the White House.

In Colorado, where Republican Donald Trump spoke to a gathering of conservatives on Friday, the GOP's lead among registered voters was only 13,607, down from 36,195 in January.

Why is that a problem? In 2012, the Republicans had an edge of 40,744 by the time voter registration closed. President Obama still won Colorado by 113,099 votes.

Democrat Hillary Clinton was already further along in building and deploying an effective voter turnout operation.

The boost in registered Democrats to turn out, especially with early and absentee voting fast approaching, could make it nearly impossible for Trump to compete, no matter how many other things he does right.

Joseph Catania is a Republican operative who is working for an outside group that focuses on registering GOP voters in Nevada. In an interview with the Washington Examiner, he likened the party's voter registration challenges to a black hole.

"The GOP needs to do more about this or soon it could be really bad," said Catania, co-founder of Engage Nevada. "I liken it to the event horizon around a black hole: No matter how much energy you expend trying to escape the black hole's gravity you can never get away."

In Nevada, the Democrats lead over Republicans among registered voters has spiked this year, from 47,612 to 66,096. That's an ominous sign for the GOP's prospects of winning the state's six electoral votes, even though public polls show Clinton and Trump in a tight race.

Obama defeated Republican Mitt Romney in Nevada four years ago by nearly 7 percentage points. At the time, the Democrats led the Republicans in registered voters by more than 90,000, an advantage built on a gain of almost 40,000 achieved in the months leading up the election.

In a state like Nevada, metrics like voter registration matter a great deal — enough to make the difference between losing and winning a presidential race, experienced GOP strategists say.

Ticket splitting happens, and often benefits Republicans, as it did in 2012 when Sen. Dean Heller won by about 1 point, even as Romney lost by seven. But historically, party registration in Nevada has proven to be determinative for presidential candidates.

"Registration matters more for Trump than others because he hasn't been here. He hasn't been in the community and investing time building legitimate relationships. Neither has Hillary — no candidate for president can," said a Republican strategist in Nevada who requested anonymity in order to speak candidly.

The Republican National Committee, which is essentially is the Trump campaign, is devoting resources to voter registration.

Groups like Engage Nevada, co-founded by RNC political director Chris Carr and also active in Colorado, are trying to pick up the slack and help the party compete in an activity that the Democrats have generally been more proficient at.

Some Republicans focused on voter registration, like Catania, worry that it won't be enough, considering existing Democratic advantages and the resources the Left is investing to enroll more voters.

In Colorado, the Democrats' successful, years-long drive to reduce the Republicans' registration edge is considered among the reasons the party has been dominant there since 2006.

Even in 2014, a banner year for the GOP, Cory Gardner won his Senate race by only 2.5 points.

Current Colorado polls show a tossup race between Clinton and Trump. But voter registration figures suggest Clinton has a structural edge, particularly with the switch there to all-mail voting, where every registered voter automatically receives a ballot in the mail.

"We've got an environment here that's a lot tougher for Republicans," said Floyd Ciruli, an independent pollster based in Denver.

Florida appears to be a bright spot for the party

There, as elsewhere, the Republicans trail Democrats in registration. But the deficit has shrunk since the beginning of the year, from 326,000 to 259,000. That's significant compared to 2012, when the Democrats increased their registration edge throughout the year to finish ahead by more than 557,500.

Obama beat Romney in Florida by just under 1 point, or about 73,000 votes.

The Florida Republican Party, focused on leveling the playing field for 2016, has been working with the RNC to register voters and cut into the Democrats' lead. Earlier this year, the state party announced that it had flipped seven counties from blue to red.

Based on what happened four years ago, in terms of voter registration and final outcome of the White House race, the GOP could be in a much better position this time around.

Democrats are worried, although a spokesman for the Florida Democratic Party told the Miami Herald that they're about to launch a voter regisration drive.

Republicans are optimistic that they might win Florida's 29 electoral votes for the first time since 2004.

"We've been on ground since 2015 creating an infrastructure that we've now accelerated," said Wadi Gaitan, a spokesman for the Florida GOP. "When you look at the math, in 2012, Obama won by 70,000 votes or so and that was with larger voter registration advantage. We've now decreased that by a lot."