With a Republican who appeals to moderates and Democrats, and a Democratic frontrunner who has united so many different groups of voters behind him, the window of opportunity for a pragmatic, independent challenger closed.

New York City Mayor Michael Bloomberg, who long toyed with a run for the White House as an independent, says he will not seek the presidency.

“I listened carefully to those who encouraged me to run, but I am not - and will not be - a candidate for president,” Bloomberg, 66 and officially an independent after bolting the GOP, wrote to the New York Times.

The billionaire and popular mayor certainly could have financed a run and gained traction quickly - against certain kinds of opponents.

But having to contend with John McCain and Barack Obama - two very popular candidates among independents, first-time voters and people in the middle of the political spectrum - quashed a Bloomberg bid before it began.

Michael Bloomberg will not seek the White House as an independent.

“The very appeal that Bloomberg would have brought to the race is the very appeal that [John] McCain and [Barack] Obama have for a lot of voters,” Time Magazine editor Mark Halperin said on CNN’s American Morning.

In his editorial, Michael Bloomberg pledged to “steer the national conversation away from partisanship and toward unity; away from ideology and toward common sense; away from sound bites and toward substance.”

Some speculated that Bloomberg, a former Democrat and Republican, would team with Nebraska Sen. Chuck Hagel on an independent ticket.

Unlike that of Ralph Nader, a Bloomberg candidacy would have broad appeal and be taken very seriously, but also unlike Nader, Bloomberg is only interested in running if he can win - and he simply deemed that impossible.

Bloomberg could still play a role in the race if he makes an endorsement. He is good friends with McCain, but he also likes Obama.