It’s no secret that former Massachusetts governor Mitt Romney has a narrow path to win the presidency this fall. Nowhere is that reality more apparent than when examining the electoral map on which Romney and President Obama will battle in November.

A detailed analysis of Romney’s various paths to the 270 electoral votes he would need to claim the presidency suggests he has a ceiling of somewhere right around 290 electoral votes.

While Romney’s team would absolutely take a 290-electoral-vote victory, that means he has only 20 electoral votes to play with — a paper-thin margin for error.

Romney’s relatively low electoral-vote ceiling isn’t unique to him. No Republican presidential nominee has received more than 300 electoral votes in more than two decades. (Vice President George H.W. Bush won 426 electoral votes in his 1988 victory over Massachusetts Gov. Michael Dukakis.)

By contrast, Bill Clinton in 1992 (370 electoral votes) and 1996 (379) as well as Obama in 2008 (365) soared well beyond the 300-electoral-vote marker.

Much of that is attributable to the fact that Democrats have near-certain wins in population (and, therefore, electoral-vote) behemoths such as California (55 electoral voters), New York (29) and Illinois (20).

The only major electoral-vote treasure trove that is reliably Republican at the moment is Texas, with its 38 electoral votes. So while George W. Bush won 30 states in 2000 and 31 states in 2004, he never came close to cresting the 300-electoral-vote mark in either race.

Bush’s two successful races, and the map on which he built them, are quite instructive when trying to understand Romney’s narrow margin for error this fall.

In 2000, Bush won 271 electoral votes — one more than he needed to claim the presidency. In eking out that victory, Bush not only carried the South and Plains states with a near sweep but also claimed wins in swing states such as Nevada, Colorado, Missouri and the major electoral-vote prizes of Ohio and Florida.

If Romney was able to duplicate Bush’s 2000 map, he would take 285 electoral votes — thanks to redistricting gains over the past decade.

But to do so, Romney would need not only to win the five swing states mentioned above — with the exception of Missouri, all of them are considered tossups (at worst) for the president at the moment — but also hang on to states such as North Carolina and Virginia where Bush cruised 12 years ago. (Obama carried both states in 2008.)

In 2004, Bush won reelection with 286 electoral votes, losing New Hampshire from his 2000 map but adding wins in Iowa and New Mexico.

Under the 2012 map, Romney would win 292 electoral votes if he replicated the Bush 2004 victory. But New Mexico seems like a very tough place to win — not to mention the fact that he would again need to carry Ohio, Florida, Colorado and Nevada as well as North Carolina and Virginia.

Now, the good(ish) news for Romney is that if he has a low ceiling, he also has a relatively high floor.

Sen. John McCain (R-Ariz.) won 173 electoral votes in 2008. If Romney carried those same 22 states under the 2012 map, he would win 180 electoral votes.

Add Indiana, which McCain lost but which will almost certainly go for Romney in 2012, and the former Massachusetts governor’s electoral floor sits at 191.

Given the narrowness of his electoral map window, the key for Romney this fall is to win in places that Bush, McCain and other Republican nominees over the past two decades have struggled to make inroads. No Republican has carried Pennsylvania (20 electoral votes), Michigan (16) or Wisconsin (10) in any of the past five elections, for example.

Win even one of them and Romney has a bit more flexibility when it comes to getting to 270 — and beyond.