While the pool of Democrats vying for the party's presidential nomination is among the largest and most diverse in the history of the United States, President Donald Trump faces a much smaller cadre of challengers for the Republican ticket in 2020. Currently, only one opponent remains in the race against Trump, in contrast to the now 11 contenders in the Democratic field, which one had more than two dozen candidates.In a statement in April, the Republican National Convention said that the Republican Party is firmly behind Trump and "any effort to challenge the president's nomination is bound to go absolutely nowhere", prompting criticism that Republican leaders are making it impossible for another candidate to succeed. Here is a look at the now sole Republican challenging Trump.Bill Weld, 74Bill Weld is a former Massachusetts governor who served two terms from 1991 to 1997. He ran unsuccessfully for vice president in 2016 as a Libertarian. He was the first Republican to challenge Trump for the Republican ticket. Weld, a party outsider with fiscally conservative but socially liberal views, has been a persistent critic of Trump, framing his challenge to the president as an act of duty. "I really think if we have six more years of the same stuff we've had out of the White House the last two years that would be a political tragedy," he said on CNN upon announcing his run in April. "So, I would be ashamed of myself if I didn't raise my hand and run."Weld is running on a platform of "respect for rule of law", "responsible fiscal restraint", and "ensuring and protecting individual liberties for every single American", according to his campaign website, and is targeting Republicans who embrace libertarian values of small government, free markets, and personal freedom.The former governor has promised to cut waste to reduce taxes. He is pro-abortion rights and has called for immigration policies that give broader access to visas and working permits for immigrants. He has previously supported bans on assault weapons and supported overturning California's ban on gay marriage. He has said climate change is real and caused by humans. Bill Weld (Jeff Kowalsky/AFP)In foreign policy, Weld has strongly opposed nuclear proliferation without oversight and has supported rejoining the 2015 Iran nuclear deal. He has said he would withdraw US troops from Afghanistan. In an interview with MSNBC's Morning Joe programme in September, Weld said Trump was guilty of "treason" for pressuring Ukraine to investigate the son of political rival, Joe Biden. He added the penalty for treason is "death". Dropped outMark Sanford, 59Former US Representative Mark Sanford dropped his long-shot bid to challenge Trump for the Republican presidential nomination late last year. Sanford attributed his decision to end his bid to remove Trump as the Republican nominee on the inability to attract attention to his key issue, the national debt.“I am suspending my race for the presidency because impeachment has made my goal of making the debt, deficit and spending issue a part of this presidential debate impossible right now,” Sanford said in a statement.Joe Walsh, 57Joe Walsh, a former US representative and conservative radio talk show host, dropped out of the race after the Iowa caucuses in February, saying he would support a Democrat in November's election. "I can't stop him (Trump) in a Republican primary, but I can sure do my level best to try to stop him ... by bring people together," Walsh told CNN when announcing the end to his presidential bid. "Any Democrat would be better than Trump in the White House," he said.