Show-stoppers are issues that fit two criteria. 1) The position that the candidate is running for can have a significant effect over the issue in question and 2) the issue in question must be one that is so important as to threaten the very fabric of the society over which the position oversees.
1) The Mayor does not control the borders. At worst case, the mayor's office could only have a minimal effect on the overall illegal immigration issue. There are times when a candidate's position on a single issue is enough to justify a vote against him, but the issue of illegal immigration, as concerns the mayoral race, is certainly not one of them. It would be a much more important issue if the office in question was President, Congressman or Senator.
2) Although illegal immigrants are a problem in Houston, the problem that they create is nominal and their presence in Houston is far from being a threat to our overall society. Compare the issue of illegal immigration in Houston, with such important issues as getting the police department under control and improving the treatment of firefighters. Those are issues that affect everyone in Houston and might very well justify changing your vote, should the candidate be on the wrong side of the issue.
Not only does the Mayor's office not have a significant impact on the issue of illegal immigration, the issue itself is much more a national issue than it is a city issue.
Now, lest you think that I am one of those robot "my party, right or wrong" Republicans, I will give you an example of a single issue that I believe is worth changing one's vote over and the positions to which it applies.
The issue is honoring the Constitution of the United States of America and the positions are President, US Senator and US Congressman. The Republicans that currently hold all four of those positions for whom I can vote (Dubya, Kay Bailey Hutchison, John Cornyn and Kevin Brady), have all actively supported and voted for (Dubya signed) legislation that is a direct and unequivocal assault on the US Constitution (the Un-Patriot Act, HSA, etc.) and for that reason, I not only cannot vote for them, I will actively campaign against them. I do not yet know for whom I will vote, in each of those positions, but it's a sure bet that it will not be the Republican incumbent. (Actually, Hutchison has announced that this will be her last term as Senator.)
In each of those cases, the two criteria mentioned above are solidly met. 1) The position in question has significant effect over the issues concerning the US Constitution and 2) there can be no greater issue than disrespect for the Constitution, that is the basis for ALL of our freedoms and thus, an attack on the Constitution threatens the very fabric of our society.
When it comes to the Houston mayoral race and Orlando, the office of Mayor has little effect on the overall issue of illegal immigration and the issue itself, though significant, does not justify ignoring other issues that are far more important to the city of Houston. Orlando may not be the ideal conservative candidate for Mayor of Houston, but he is by far, the BEST.