As Republicans begin to gear up for the 2016 presidential race, GOP insiders have one major concern that wasn't mentioned anywhere in the RNC's autopsy report.

Her name is Hillary Clinton, and Republicans worry that they won't have a shot against her, regardless of what changes the party makes before 2016.

Although most Republican strategists would not go on the record disparaging the party's chances, in private conversations with Business Insider, many savvy GOP insiders conceded that any Republican nominee would face an uphill battle against the former Secretary of State.

"If she runs, it'll be almost impossible, I fear," one Washington-based Republican strategist told Business Insider.

A New York-based GOP media strategist concurred:

"It will be extremely hard for Republicans to beat Hillary in 2016," he said, adding that the difficulties would be compounded if the party nominates another candidate who fails to ignite the conservative base.

"Should Hilary run and Jeb [Bush] be the nominee, how can you ever defend the Bush years against the Clinton years?"

Recent polling validates these concerns. A new Quinnipiac survey released Thursday found that Clinton would beat both Bush and Florida Republican Sen. Marco Rubio in a head-to-head match up in the Sunshine State.

According to that poll, if the race were held today, Clinton would dominate Bush, 51 percent to 40 percent, and beat Rubio 52 to 41 percent. Her favorability rating is also much higher than either of the home-state Republicans — 62 percent of Florida voters view Clinton favorably, compared to 50 percent for Bush and 41 percent for Rubio.

On a national scale, Clinton left office with record-high favorability ratings — a January Washington Post/ABC News poll found that a full 67 percent of Americans viewed the outgoing Secretary of State favorably.

In addition to her popularity, Clinton's national name-recognition and extensive fundraising network — not to mention her experience running a presidential campaign — make her a formidable opponent to both Democrats and Republicans.

But the biggest problem for the GOP is that if Clinton decides to run, the Democratic primary will likely turn into a de facto contest to pick her running mate, giving the party a big head start in the general election. In the meantime, Republicans will likely have to slog through a contentious primary with what is shaping up to be a very crowded field.

"Hillary's path forward will likely influence the breadth of Republican field as much as the Democratic," a Washington-based GOP strategist told Business Insider.

Still, some Republicans are holding out hope that a candidate will emerge who can beat Clinton.

"Republicans can beat Hillary, but no one has emerged yet," one New Hampshire-based GOP operative said. "But three years is a long time."

Michael Duncan, a Texas-based Republican consultant, concurred:

"Clinton brings the stature and fundraising prowess of someone who's been in Washington for 20 years. But I also think that advantage is mitigated by voters' widespread mistrust of our established political leaders," Duncan said. "Republicans should look to the 2008 primary as a guide and nominate a fresh-faced outsider [who] can create the contrast Barack Obama did. Senators Rand Paul and Ted Cruz are two of those transformative figures."