Walter Shapiro argues that there is still a chance for new Republican presidential contenders to enter, but he acknowledges that there isn’t that much time left:

Make no mistake, some deadlines matter in presidential politics like the precise dates for getting on primary ballots. Although the entire GOP primary calendar is in flux, the best guess is that the deadline for filing for the Florida primary will be Halloween, with other early states like New Hampshire following soon after. While Henry Cabot Lodge did win the 1964 New Hampshire GOP primary on a write-in vote, resorting to that pencil-based strategy would be a daunting price for, say, Christie to pay for his indecisiveness.

The filing deadlines are one of the most important barriers to someone entering the race this late. Besides Florida and New Hampshire, South Carolina and Michigan’s deadlines are on November 1 and 15 respectively, and New Hampshire’s filing deadline may be moved earlier to accommodate an earlier primary date. There might still be enough time to meet those deadlines, but that assumes that new campaigns know what they’re doing. Even Perry’s campaign had a lot of catching up to do when he entered the race in August, and learning all the requirements is a time-consuming process:

Though the campaign of Texas Gov. Rick Perry is making gains in the polls in just its second week of operation, it is scrambling to catch up with its longer-running rivals on matters of ballot access. “There are 56 different states and territories, and all 56 have completely different rules,” said Ryan Price, who handled delegates and ballot access for John McCain’s 2008 primary campaign. “Even when you take away the deadline aspect, just to do all that research could take weeks, if not months.”

Even assuming that a new campaign could meet all of the requirements for ballot access in the early primary states, the primaries are becoming almost as front-loaded as they were four years ago. Thanks to Florida’s early date, the caucuses are probably a little over two months away, and New Hampshire will be in mid-January. Time is against any new candidates. It is likely that anyone jumping in at this stage will miss filing deadlines or will have so little time to campaign and raise funds that he will never be seriously competitive.

Update: The Wall Street Journal reports that there may be even less time before Iowa and New Hampshire:

In the wake of the announcement, New Hampshire Secretary of State William Gardner said his state could hold its first-in-the-nation primary before the end of the year in order to uphold tradition [bold mine-DL].

That would put the first nominating contest—the Iowa caucuses—in mid- to late-December because the state schedules the caucuses eight days ahead of the New Hampshire primary. Iowa GOP Chairman Matt Strawn issued a statement Friday to say they will name the date once New Hampshire schedules its primary.

Second Update: The N.H. Secretary of State has moved the filing deadline to October 28.