A late entrant is usually someone who needs to be given an excuse to get into the field. Most commonly, it's a candidate who's new to the national scene, needs to show there's no one else on the horizon who's better, and wants to tack on additional months of experience in office. Barack Obama had only been a U.S. Senator for about two years before his run for president, and he wanted to wait as late as possible. So he continually denied he was running up until he was ready to announce.
The same could be true of Chris Christie and Ryan; they're both new to the national scene. As for Perry, a late entry would also make more sense: Texas reelected him last November, and starting a presidential run immediately afterward would have been a bit awkward.
This leaves us with Giuliani. He has no particular reason to run later than the other candidates in the field. If people wanted to support another Giuliani campaign, they already would have. This suggests Giuliani will not run. He would need to win New Hampshire, where competition will be stiff. In 2008, Giuliani skipped the four early primaries to focus on Florida. This was a dreadful mistake, but it was also due to the fact that he couldn't get traction elsewhere. There's little reason to think he would gain more traction this time. If anything, voters will be even less receptive to a candidate branded as "the anti-terrorism guy".
What about Ryan? In one way, it would make sense. A late-entry candidate should be someone who, like Christie, is acceptable to both the Tea Party and establishment wings of the Republican Party. On the other hand, Ryan is seen as more of a "policy wonk" (i.e. a nerd) than someone who fires up a crowd. A late entrant should be someone who has a more charismatic personality than Tim Pawlenty, who is already filling the "acceptable to both wings" role. In addition, Ryan is only a U.S. House Representative, so his perceived electability will be lower.
Ryan is also best known for his failed attempt at reforming Medicare this year. Most voters have only heard of him in that context. His attempt did not fail simply because it did not pass; after all, a Democratic president meant it was guaranteed not to pass. It failed because it did not persuade voters. The Republican Congress of the 1990s succeeded with welfare reform because it was so popular that it made President Clinton look bad for vetoing it. Clinton was then forced to change his mind and sign--even embrace--welfare reform. Ryan's supporters called his Medicare reform "brave" and "adult," but they did not call it "compelling," "ingenious," "effective," or "popular." There is no such thing as "brave" legislation; it's either persuasive or not. Ryan's plan did not convince voters, so it must be counted as a failure.
As for Perry, we have already seen that simply being a Southerner does not guarantee him an opening. However, Perry is more likely to enter the race than either Giuliani or Ryan. He can more easily claim to be a "stronger version of Pawlenty." On the other hand, his being another governor of Texas (and tied to George W. Bush) will be a big turn-off, even to Republicans. He might still be able to defeat someone like Pawlenty, but it will depress Perry's poll numbers and make him less likely to enter the race. Simply put, few people are clamoring for Perry to run. He doesn't have a base, even though he's had ample time in politics to develop one. He may want to run, but he won't be tempted unless his poll numbers rise.
