Former Texas Gov. Rick Perry said Thursday that he endorses Donald Trump as the Republican presidential nominee and left to door open to being his running mate, according to CNN Perry, a former Republican presidential candidate, once criticized Trump and then endorsed presumptive Republican nominee's chief rival Ted Cruz after making an early exit from the race."He is not a perfect man," Perry told CNN of Trump. "But what I do believe is that he loves this country and he will surround himself with capable, experienced people and he will listen to them. … He wasn't my first choice, wasn't my second choice, but he is the people's choice."Perry told CNN that he has not spoken to Trump in six months but would not turn it down if offered the vice presidential spot."I am going to be open to any way I can help. I am not going to say no," Perry told CNN.Perry's endorsement comes as other top Republicans have struggled to accept Trump as the Republican nominee. In comments on CNN's "The Lead with Jake Tapper" Thursday, House Speaker Paul Ryan said he was currently withholding support for the real estate mogul turned politician."I'm just not ready to do that at this point. I'm not there right now," Ryan said on the cable news program. "I'm not there right now."Trump shortly fired back at Ryan, according to ABC News , with statement calling on Ryan to work with him."I am not ready to support Speaker Ryan's agenda," Trump said. "Perhaps in the future we can work together and come to an agreement about what is best for the American people. They have been treated so badly for so long that it is about time for politicians to put them first."Mitt Romney and John McCain both announced they plan to skip the Republican National Convention in Cleveland, along with former presidents George H.W. Bush and George W. Bush, according to the Washington Post Perry, who also ran for president in 2012, served as Texas governor from 2000-2014 and before that lieutenant governor under George W. Bush before he became president.