By Dan Burns
NEW YORK (Reuters) - Donald Trump's prospects of winning the Republican nomination for the White House have slid sharply in online wagering venues, and bets that Republicans could have a fractured convention or that Trump may run as a third-party candidate were increasing.
Chances of Trump claiming the Republican mantle have fallen hard to 69 percent after cresting at 86 percent this week following his winning seven of 11 states in Super Tuesday voting, according to PredictWise, which aggregates betting on multiple venues into an implied probability.
The billionaire businessman is still the prohibitive favorite, but it was the steepest slide in his imputed chances since his second-place showing in the Iowa caucuses on Feb. 1.
Trump faces withering attacks from rivals Ted Cruz and Marco Rubio and from 2012 Republican nominee Mitt Romney, who labeled him a danger to the Republican party and the United States.
On PredictIt, a site aggregated into PredictWise probability, Trump's chances were 70 percent early Friday, up slightly from late Thursday when they fell to 68 percent, their lowest since Feb. 23. PredictIt is operated by Victoria University in New Zealand.
Online wagering venues such as Ireland's PaddyPower, which express candidates' prospects as traditional sports-betting odds, show Trump losing luster as Rubio and Cruz gain steam. PaddyPower's odds show Trump as a 2-to-5 favorite compared with a 1-to-7 favorite right after Super Tuesday.
Odds on Rubio, a U.S. senator from Florida, narrowed to 4-to-1 from 5-to-1 on Tuesday, and odds for Cruz, a U.S. senator from Texas, were 9-to-1 from 12-to-1 earlier this week.
Intense squabbling among Republican candidates and efforts by establishment Republicans to block a Trump nomination at this summer's Republican National Convention boosted bets that former Secretary of State Hillary Clinton, the favorite for the Democratic nomination, would win the Nov. 8 general election.
Clinton's prospects of winning a head-to-head contest against Trump shot to above 60 percent earlier this week and stood at 57 percent on Friday, according to PredictIt. Trump's chances of trouncing Clinton sank to 27 percent on Friday from 39 percent in late February.
Republican infighting is fueling betting that Republicans will have a so-called brokered convention, which occurs when no candidate wins a majority of delegates before the convention, or that Trump will run as a third-party candidate.
Chances for a brokered convention closed at 40 percent on Thursday on record volume in betting on PredictIt after starting the week at 23 percent. They were 39 percent early on Friday.
The last brokered U.S. major-party convention was in 1952 when Democrats nominated Illinois Governor Adlai Stevenson on the third ballot. The last brokered Republican convention was in 1948, when New York Governor Thomas Dewey won in three ballots. Neither candidate won the presidential election.
Chances that Trump would run as a third-party candidate have ticked higher, but it remains a long-shot in betting circles. PredictIt put the chances of him running as an independent at 17 percent on Friday, from 14 percent on Thursday and 10 percent a week ago.
(Reporting by Dan Burns; Editing by Toni Reinhold)