July 27 (Reuters) - Colombian lawmakers who had supported a constitutional change to allow popular President Alvaro Uribe to run for a third term in 2010 concede the bill is not likely to pass.

Uribe, popular for his crackdown on leftist rebels, has not said whether he wants to run again but his government has lobbied strongly for the measure.

Politicians from across the political spectrum say the proposed referendum on whether Uribe should be allowed to run is unlikely to be held after lawmakers from outside the Uribe re-election camp were elected this month to key House and Senate leadership positions.

Should Uribe be barred from campaigning, here are some of the candidates lining up for a shot at the presidency:

JUAN MANUEL SANTOS - Colombia's high-profile former defense minister and Uribe ally, Santos is often touted as the most likely politician to be anointed as Uribe's successor. His ties to the successful security campaign against leftist rebels and experience in the cabinet in defense and finance posts could make him the favorite. But many believe Santos, from an elite Bogota family, lacks enough connection with voters to win.

SERGIO FAJARDO - A youthful-looking former mayor of Medellin who studied mathematics in the United States, Fajardo is popular in that city for having cut crime. After he left the mayoralty, Fajardo briefly joined a national radio station as a commentator, which raised his profile. The math teacher-turned-politician is seen as a new face in politics and is neither staunchly aligned nor opposed to Uribe.

NOEMI SANIN - A former presidential candidate herself, Sanin quite her job earlier this month as Colombia's ambassador to the United Kingdom in order to run for the presidency. During a business forum last year, Uribe said he believed she was one of the most "competent" supporters of his policies. She has been out of the national spotlight for a while.

FELIPE ARIAS - A former agriculture minister, Arias is a staunch defender of Uribe's security and investment policies and wants to run as the Conservative party candidate if Uribe steps aside. Known as "Uribito" (Little Uribe), Arias combs his hair like the president, wears the same style of glasses and speaks with the same Medellin accent. But many wonder if the younger version is ready to fill his former bosses' shoes.

GERMAN VARGAS LLERAS - A former senator and leading member of the Cambio Radical party allied with the president, Vargas recently returned from studying overseas and has been among those mentioned by Uribe as a possible standard-bearer for his policies.